Saloth Sar used a few different aliases during his time as a revolutionary and leader. For instance, while studying in France he wrote an article for a communist magazine under the name khmer Daeum (Old Khmer). In 1970 he changed his alias from Pouk to Pol. He would later adopt the second part to Pol Pot. This took place before the leadership headed south from their base in the northern heavily forested region of Ratanakiri. The 'why' of this is apparently a little unclear, as is the precise meaning of the new name. Here is a section from Philip Short's excellent biography of Pol Pot - History of a Nightmare:

On the eve of their departure, Sar called the leaders of the group together. 'Now we are going down to the plains,' he told them. 'Today we will all change our names. I will no longer be Pouk. From now on I will call myself Pol.' He then gave the others new aliases, which he had chosen himself. His Jarai assistant, Phi Phuon, who was with him that day, received the name Cheam. It was a rite of passage, opening the way from one existence to another. Khmer men took new 'names-in-religion' when they became monks; Khmers Rouges did so when they took on new responsibilities.

So that gets at the why the name was changed, it was a moment where the movement was taking a step forward and perhaps metaphorically the name changes reflected this movement too. Short also explains the language aspects of this name change;

Pol - or Pol Pot, as he would later call himself, following the custom of adding an euphonic monosyllable, in the same way that Vorn made his name Vorn Vet and the eldest of the Thiounn brothers was called Thiounn Thioeunn - never explained why he had chosen his new name. However, the Pols were royal slaves, the remnants of an aboriginal people, 'noble savages' in Rousseau's terms and it is tempting to see in the soubriquet a sardonic reference to Sar's alliance with Sihanouk which made him, metaphorically, the 'slave of the King'.

This is in reference to the coup which had removed Sihanouk from power and placed him, in a figurehead role, as the face of the anti-Lon Nol regime movement. So that is one potential explanation, however Short finishes the paragraph saying "Whatever the reason, for the next decade Sar would be known as Pol."