I'd question a few premises in your question:

1. On what do you base calling the Dutch colonial empire "liberal"? Slavery, for example, was only abolished in 1863, long after Britain and France. If you're referring to the Dutch "ethical policy", this was only introduced in 1901, long after the end of Dutch influence in South Africa.
2. You mention that South Africa was heavily influenced by the Dutch. While most of the early settlers, starting from 1652, were Dutch, the Dutch had little interest in the colony beyond as a port to supply passing Dutch ships. The early Dutch settlers were supplemented by large numbers from other countries, such as the French Huguenots, and offspring from slaves were incorporated into the society (the well-known governor, Simon van der Stel, was of mixed-race origin). The British took final control in 1806, long before the introduction of formal apartheid in 1948. Between those two dates, a lot happened that had a large impact on the future history, far more than the early Dutch influence.

A* Dutch, Liberal??? You seem to be confusing modern Holland with its 19th century version.

This however is how South Africa came to become Apartheid. The Dutch presence in SA can be traced to a way-station(place for ships making the trip to Asia to restock) established by the Dutch East India company, due to the company thinking about its bottom line they encouraged the original settlers to fend for themselves, by farming and such.

Fast forward a couple of years, the "burghers"(Boers) as they came to be known had assigned to themselves tracts of land, to be passed on from Father to eldest Son,other Sons were encouraged to head off and acquire more lands for themselves, obviously the indigenous tribes, the San for example did not like that but a lack of guns didn't help them state their point. Due to the Boers often time having a lot of land, they needed a labour force. This was originally found in immigrant Indians but was gradually replaced with Black Slaves.

Britain had maintained a nominal presence in SA primarily in the Cape, but as always in order to secure her shipping routes she moved to bring the Boer territories under her rule. She was able to do so but this is where it gets interesting. Britain brought with her rules certain things the Boer found "repulsive".

• A relatively liberal attitude in comparison to the Boer in the treatment of Blacks. Fueled by Christian Missionaries drawing attention to the plight of the Blacks, slavery was abolished, they were now called apprentices who had to be paid a wage.

• Blacks were given the right to sue over mistreatment by their Masters. There is even a story about a Boer who rather than suffer the "indignity" of being dragged to court by a servant resisted arrest and was shot, this contributed led to a Boer riot.

• The British tried to suppress Boer culture by having all Court Records and Public Schools operate in English.

The Boer felt under siege, they felt outnumbered in "their" land, under siege from a foreign Power determined to challenge all they stood for, their only solution was to leave. Which they did in a movement known as the great trek. Thanks to the Mfecane(a series of Wars in the Ngoni region of SA) which led to migrations by tribes seeking to escape Zulu conquest, the trekboers found a lot of empty land, they "trekked" into the interior, beat the war-weary tribes left from the ravages of the Mfecane and promptly declared the land theirs by right of conquest.

But Britain just could not resist trolling, because unluckily for the Boer, Gold and Diamonds were just being discovered and Germany had decided to get in on the colonies race. So Britain had to occupy the Boer territories again, now keep in mind that this was the greatest empire the World had ever seen, they had by now forgotten what it was like to lose. Which is why they were shocked when the Boers started defeating them and the War started taking more money than was budgeted. Eventually Britain won at great cost, however the war had awakened a fierce nationalism in the Boer, they would not be satisfied except with a government on their own terms. "Afrika for the Afrikaans" as they would put it, eventually Britain would grant South Africa its independence as a Union which would make voting an all white affair. This would lead to the Nationalist party coming to Power which in-turn would lead to the adoption of Apartheid.

Apartheid was motivated by fear of once again being subservient to any rule that was not Boer in person and spirit.