Some were simply allowed self governance, as happened in Canada. For those that fought back, a big part of that was caused by the hierarchy of race, class, or religion established by them, like in China. But let's go with the big reason the Empire fell apart: money.

The "First British Empire," when they held their North American colonies, by 1750 had been a result of 150 years of investment and hard work settling there. Colonial wars occured, and quite frequently, with there being four notable "Indian wars," the fourth being what we Americans call the French and Indian war but what most of the world calls The Seven Years' War. Just before it started, tensions in the Allegheny region and New England were increasing over land rights between the French and English. The French continued to settle west of the Alleghanies, notably going to the convergence of the Allegheny and Monogahella Rivers, where Pittsburg is today, and building Fort Duquesne. Fighting in the area broke out by 1754 and relations continued to worsen. In 1755 the British colonists had suffered numerous defeats at the hands of the French and their allied tribes in the Ohio Valley Region. French settlers were even expelled from New England by the governor of Massachusetts, fearing they would become French supporters. In 1756, England declared war on France as global tensions boiled over.

William Pitt, also known as "The Great Commoner," was a British statesman in the early to mid 1700s. He had a little influence in the third Indian war 20 years earlier (War of Austrian Succession globally, the N. American sub-portion of it is called the War of Jenkins' Ear and ended dispute over the Spanish Floridia-English Georgia territory line) but really took charge in the Seven Years' War, saying "I am sure I can save this country, and nobody else can." He did. He utilized resources to boost the Royal Navy and used them heavily against the French of the coast of England. He updated the army and sent them to the N.A. colonies. He sent subsidies and and some reinforcements to the Prussians to maintain the mainland Europe war. He aided in the battle against the French East India Company for control of territory in India. By 1759 it was said in England that "Our bells are worn out threadbare with ringing for Victories." They had achieved major victories over the French in India, Canada, and Europe. They had also defeated an uprising (of sorts) fueled by the Dutch Trading Company in India. By 1763, the war ended and a massive empire would be the result - they would gain land holdings including Florida, all land from the Mississippi River to the Appalachian Mountains, and most of Canada (or "French Canada"). Pitt had built a massive empire, doubling the land holdings in North America alone, and had done so at great expense to the country. As a result, for the first time since they were established, the colonies in America would be expected to contribute to the wealth of the Empire. Trade, particularly smuggling, was more heavily regulated. In 1765 the Stamp Act passed and the first flickers of the fire that became a revolution started to grow. Within a decade the Intolerable Acts would pass as a result of colonial belligerence (things like the Boston Tea Party), which was the colonists' response to the stamp act. Two years after the Intolerable Acts, in 1776, America would declare independence. England would, of course, lose those territories officially in the 1783 Treaty of Paris; the sun was setting on the First Empire.

Historians disagree as to when the First Empire ended and even moreso if the second had been started already. Bit it did end. The Empire was by no means gone, and still held vast territories, but would not be the same in philosophy. While authority may have defined the First Empire, world trade would define the second. In addition to the end of several trade monopolies, the Second Empire saw the ban of slavery in 1807 and even more European wars, including the French occupation of the Iberian Penninsula just a few years later that prevented Spain and Portugal from properly tending to their colonies. This gave England an opportunity to establish trade deep within those colonies and they utilized it. Their purpose in the Americas would forever be profitable trade past this point, including protecting Atlantic/Europe bound trade ships from former Spanish or Portuguese South American colonies fighting for their independence. They let it be known that independence in the America's would not be obstructed by Europe on their watch and held naval posts there to help enforce it. By the 1830s the Spanish were out of South America and North American cotton was pouring into English textile mills. The number one importer of English textiles at that time was the former Spanish and Portuguese colonies in South America. The British were mostly done with colonialism in the America's.

Backing up a step, the industrial revolution and cost of slave rebellion supressions weighed heavily on the effective value of slave dependent goods, leading to a reduction in the slave trade (there was also a growing global abolition movement). It's hard to say if it was market dynamics or human good decency that ended it, and an arguement can be made for both. And most likely both played their parts. For instance there was discussion of limiting slave importation to help regulate the cost of sugar, which was an issue for some time. The market was seemingly demanding less slaves to stabilize itself in some sectors, which is in its essence a basic wage negotiation (showing the practical economic effectiveness of slavery was diminishing). In any event, after slavery was banned by England, though not the first country to do so, they began to enforce the ban of the slave trade on the high seas. This gave them all the more reason to establish posts in the Americas, and also to do so in Africa, colonies that were so by definition and used for trade expansion but not in the same ways as colonies they had built previously at Jamestown, Charleston, or St Simons/Frederica. At least not with the same primary intention of establishing massive and continually profitable production/income sources. Sierra Leone was used as a Royal Navy base for the Atlantic and championed for free slaves, similar to Liberia, with the "cargo" of captured slave ships being released there as free souls by the Royal Navy (Gold Coast and Gambia were also Royal Navy bases).

In the East, the East India Company continued their occupation of India with support of the Royal Navy and massive gains in land and control occured. They had moved settlers into Australia and New Zealand starting in the late 18th century, satiating any craving they still had for the old American style colonies and extrapolating wealth from them, namely in gold from Victoria. Despite gaining those above, almost all of the British colonies gained in the 19th century would be done through war and treaty, including in south Africa and Hong Kong. The steamship and telegraph played huge rolls in maintaining and expanding the empire, allowing words and goods to move quickly. With this technology also came the need for a global network of coal stops (as well as cabling). The Empire continued to blossom as a mighty trade network supported by the Royal Navy. By the end of the 19th century, internal warfar in their colonies and a race for expansion by almost all European powers had created and dispersed relatively stable colonies in the Pacific and Africa. By 1900 even America had become a small empire, freeing Cuba and gaining Puerto Rico (ending Spanish authority in the Americas once and for all) as well as gaining Guam, The Philippines, and Hawaii.

Canada had sought more self governance (or the provinces of Canada, more accurately) and achieved such in the mid 1800s with the Act of Union in 1840 and Quebec Conference in 1866, gaining some political sovereignty but still as subjects to the crown. Australia and New Zealand thought that they should, too, and worked to accomplish the same goal by the late 19th/early 20th century. As long as trade remained good, Parliament didn't seem to mind giving local control to royal subjects in these well behaved foreign lands.

WWI happened and England invaded more territories, even convincing the Japanese to invade the German beer making colony in Qingdao, China. By the end of that war the Empire was bigger than it had ever been before, both in land and population - and was doomed. Australians and New Zealanders, feeling they earned it at Gallipoli, saw their right to full independence and became more vocal. The urge similarly grew in India. Many colonists pushed back at this point, seeking independence. The 1931 Statute of Westminster essentially allowed Australia, New Zealand, South Africa, Newfoundland, and Canada their own government without interference or dictation from Parliment, finally granting independent authority to some of the (former) colonies.

England would hang on to many of her territories in force and spirit until the end of the second world war. Within a few short years, the rule of the crown would drop by over 600,000,000 citizens. Europe lay in ruins. England was bankrupt. She had lost her world power standing as the war started and largely just had no more capacity for colonial rule. India left the Empire in 1947, the Suez Canal (which they largely owned) caused more crisis in the 50s. The dominoes continued to fall as African colonies, like Sierra Leone, finally gained liberty.

In addition the global economy had started which ended regional sourcing. Now ingredients could be shipped from anywhere, assembled, and sold. There was little reason to hold far away lands. Colonialism was dead, but imperialism remained (and in ways still does) in the control of financial markets and lending across the globe.

This is really a case of 'asking the wrong question' I think. You need to ask 'what was the Empire for?' And that was for trade. When wars were common enough that traders and fledgling colonies needed to be protected by the home nation, and with enforcement of the Pax Britannica, then it made perfect sense for 'hard power' in the in the form of far-flung naval bases and garrisons. With the post WW2 peace, the increasing demands for independence as a result of the collapse of European supremacy as the US and the USSR become the world's superpowers, and a poorer Britain which can't afford to enforce its will anyway, they're no longer really needed. Post war, 'soft power' becomes the norm, and physical control of an area is no longer a requirement for trade to function - Empires are now obsolete.

The new post-war weakness of Europe was particularly demonstrated during the Suez Crisis, where international political pressure forced Britain and France to abandon their attempts to exert control over the canal zone.

In your examples there are several different reasons happening - some because the former colony is now large enough to be independent, some because the mood of the time means Britain can't and/or will no longer attempt to rule directly. In the case of Hong Kong it was because with the refusal of China to re-lease the New Territories, it would have been impossible for Hong Kong to survive independently from the mainland.