Well, flap extension can reduce the angle of stall. Slat extension reduce it.
So if the flaps were to deploy faster, the flaps decrease of angle of attack could win against the slats increase. Until the flaps are fully deployed, and now the angle of stall will increase further than previously.
I'm with KayPam on this (because I came to the same conclusion a few years ago). The effect is particularly noticeable if you have full speedbrake deployed when going from 1 to 2.