There are a few reasons given by historians (as well as charges against the surviving leaders of the Communist Party of Kampuchea, or CPK) for the forced evacuations of all major towns and cities in Cambodia once the civil war ended in April 1975 with the collapse of the nation's capital, Phnom Penh. Former leaders have also given reasons as to why this directive was given. This answer will circle through the events that you refer to, and the reasons given at the time, before assessments from historians and the Khmer Rouge tribunal. That should give you and people who maybe aren't aware of the details a slight intro before getting to the 'reasons' for one of the largest forced migrations of people in history.

By early 1975 the Khmer Rouge (a communist armed movement supported by China that was originally an offshoot of the Vietnamese Communist Party) controlled nearly two thirds of Cambodia, they had already begun small collectivised communes in some areas under their control and characteristics of their rule during the revolutionary years (1975-1979) could be seen in these areas. For instance, in 1973, collectivised labour/farming and the uniformity of dress (usually a plain black 'pyjama') had been adopted in areas of the Northeast. Restrictions on religion were also enforced.

In April of '75 Phnom Penh had been besieged for months, with mines blocking merchant ships from delivering food down the Mekong and random shelling of the city causing chaos. It is important to note that the population of Phnom Penh had also increased dramatically during the civil war as refugees from the countryside fled to the capital to escape poverty, destruction or the advancing communist forces. Most estimates place the population of Phnom Penh in April to be somewhere around 2 million, but I have seen higher. This was at least double the 'peacetime' population. Food was increasingly scarce and aircraft was relied upon to deliver rations - these shipments were also under threat of anti-aircraft shelling.

On the 12th of April the US Embassy closed and most major diplomats and all US staff fled the country - The US was effectively 'bailing' from Cambodia and Southeast Asia in general after an extremely costly and altogether embarrassing display that gave the communist forces in Cambodia (even though it was primarily the Vietnamese who had secured this 'victory' against the US) a huge propaganda opportunity as their forces began encircling and eventually entering the capital and crushing any resistance from left over Khmer Republic forces (the name given to the short lived country that had fought the civil war against the Khmer Rouge following the coup against the former ruler Norodom Sihanouk).

Once the Khmer Rouge entered the city they forced all opposition forces to surrender and gathered their weapons at road blocks. Columns of soldiers, some on tanks and vehicles, streamed down the wide boulevards of Phnom Penh as onlookers cheered them on - celebrating with white flags - thinking peace had finally broken out in a country marred by a war that killed perhaps as many as 500,000 people.

It became clear that this was for many the beginning of the nightmare as these troops, often very young, began ordering people to abandon their homes and to go into the countryside. Some people were told that they should go back to their old villages, some were told that they should simply get onto a nearby road and keep walking, others were told that US forces were going to bomb the city and that they needed to evacuate for at least 3 or 5 days, but that they would be able to return after. During this time any high ranking officials of the Lon Nol regime who hadn't fled the country were round up and shot, as were large amounts of army personnel. Foreigners who had remained, including some international officials and diplomats from the Russian or French embassy - as well as some journalists (see the film the Killing Fields) were gathered up and eventually confined to the French embassy. One of these journalists, Jon Swaine, recalled seeing patients ejected from a hospital at gunpoint who must have only recently had amputations administered in surgery - they were forced to drag their bleeding, bandaged limbs across the road - while others were pushed on hospital beds, drips still attached. It was callous in the extreme.

The evacuations of the population of Phnom Penh took place at an agonisingly slow crawl, it took days before people were able to navigate themselves to the outer limits of the city as the roads and highways were literally choked with people, some old, some young, some on oxcart and a few on cars that soon ran out of petrol and needed to be abandoned. Imagine leaving an extremely crowded sporting event or a music festival, some kind of bottle neck scenario, but that it lasted for days... and in the middle of the hottest month of the year. There were regular checkpoints for the population as they got closer and closer to the countryside. At these points people were required to give a short biography or disclose whether they were part of the former government/army. Those who said they were part of these groups were often taken away and killed. It should be noted that this was not always the case. Some technicians actually were taken back to Phnom Penh and worked in factories, while sometimes being a former member of the army was not a death sentence... at least not yet. This reflects dynamic ways in which CPK policy was adopted in different parts of the country and I will address this in part two of this answer.

Some estimates place the death toll during the evacuation (which took weeks and even months for some people) to be at least 20,000 civilians.