It could be that they have to get in the paperwork before the 31st so it can be manually processed. So the last day to them would be a day or two before. Also the beginning of the month makes sense because you'd want to dole out a bunch in the first week so you could coast a little in the middle.
That was my first thought, but then I think we'd see a larger drop on the 30th as well, since that's the last day of the month sometimes too. Someone suggested doing a graph with "days until end of month" on the X-axis, which might be interesting.
