It strikes me that asking "Who was the last person to quit, and why?" is a big faux pas in this area. Am I wrong, or is it common/OK to ask this kind of question?
It's been my experience that very few people "quit." Separations from startups tend to be very neutral, leaving it ambiguous whether someone quit or was let go. It's not hard to read between the lines, but discussions about this are not very up-front.