I'm trying to guess their rationale. Starting with the most friendly option and ending with the most cynical:
Great job, team! Glad you're sticking around. Here's your money early. Keep up the great work.
Maybe if we give people more opportunities to accept (i.e. a longer window), more of them will.
If we can get people to commit earlier, that will help out with force management over the next few years.
If sequestration continues, it will be harder to come up with the money in FY '15, so let's move some of that bill to FY '14. That way we can still pay the remainder that accept in FY '15.
If sequestration continues, it will be impossible to come up with money in FY '15, so let's lock-in as many pilots as we can while we can.
We're going to need to raise the bonus to meet retention goals, but why pay everyone more? The "I was going to stick around anyways" crowd will bite off early on the current, lower rate. That way, when we raise the bonus next year, we know we're actually targeting the "if the bonus is good enough" crowd, thereby meeting our numbers while saving money.
My bet is a combination of #2, #3, and #4. Only #6 would really be a trick.