Each time you add a new bounty to the same question, the minimum amount of reputation that you can offer is doubled. From the Help Center:
Additionally, if you offer multiple bounties on the same question, the minimum spend doubles with each subsequent bounty (50 reputation on the first bounty, 100 reputation on the second, 200 on the third, and so on).
Your first bounty cost 50 rep, the second 100, then someone else offered a third bounty on the question, and right now you don't have enough reputation to offer the 200 you'd need for the next bounty.