Nothing, all is good and well, though you obviously suggest a different outcome.
Rule No. 1: A particle ipo of 100 frames is evaluated in the lifetime of the particle (by default).
So lets take a look at the ipo curve for GravX. It has a positive force till frame 78. So the particles are accelerated to frame 50*0,78 = 39. Then they are accelerated in the opposite direction and get slower at the end.
Rule No. 2: You can’t set the particle speed directly.
Accelerating an object is something different than settings its speed. You can only set the starting speed.