Essentially, this is to make sure that you stay at Microsoft for longer. Any starting bonus over the value of $25,000 will be split up over 2 years (half the first year in your first paycheck, the other half in your first paycheck a year after I believe).

Of course, like many things there are some discprencies or stories of new full-time employees who receive their starting bonus in the first year and have a bonus >25k - you may be able to try to negotiate getting your entire bonus in the first year but this may generally be a hard and fast rule across Microsoft.