I've been searching for an answer to that, and from what I have seen so far it looks like a combination of things that cause the candida to grow from the DMSA. If I understand what I've been reading correctly, this is what seems to be happening:
1. DMSA binds to zinc, as well as to the intended heavy metals. This is even on the label of my VRP DMSA bottle that it does this.
3. Mercury in the gut may kill the good bacteria that help keep candida in check, rather than the yeast growing on purpose to eat up the mercury. Or it may be just by the general weakening of the immune system that mercury causes that causes the yeast overgrowth. The mechanism of all this seems to be mostly guesses, from what I've read though. But I do fight back on this aspect by providing tons of good bacteria, which seems to me to always be an uphill battle.