Is this to encourage answers, since we have a kinda low answer rate, or is there some other incentive there I'm missing?
In general, I would say that it is a coincidence. There's been quite a lot of time over the last few months where there have not been any bounties, and at the time of posting there are two bounties.
We're not a huge site at the moment, so the number of bounties is probably going to range between 0 and 3 for quite a while.
I've been experimenting a little bit with bounties over the last couple of months to give a little extra attention to some unanswered questions. I generally only do one at a time. And in general, a bounty poster might be inclined to think that if there are no current bounties, this might be a good time to post a bounty. So that might contribute to the apparent persistence of one bounty.