For the hackers who broke in and gathered the numbers, large batches of fresh numbers make them the most money. They sell the dumps and the more numbers/the more recent, the more they make. So hitting a big target, gathering a huge collection, and then dumping it all at once is the most profitable. The people who buy the dumps have to worry about cards getting flagged, but not the people selling them, and the hackers dumping the cards don't really care what the people who buy the dumps have to deal with, they just care about getting the most cash they can.

Also since any release at all is highly likely to trigger an investigation, a small dump could be the last, so a big dump is the lowest risk.