It is completely random on when you get one. I've received two so far. The first pop was after turning in a crucible bounty, the second time was after a vanguard one. Clearly the the bounty type is not a factor, especially as I've turned in more vanguard than crucible bounties. Also the number of bounties is likely irrelevant as well, as the second bounty popped shortly after the first, with far fewer bounties completed in between getting the first and second. Additionally, I have yet to complete either, so completing the first bounty is not required in order to get a second one to pop. What is interesting to note is that the second time the option to select an exotic bounty popped, I only had two choices instead of three, which I had the first time.