Since some update everything floats to the top-right. Your theory seems to be pretty good.
Possibly the water particles are being affected first by the top-left section of the GPMP, which causes them to move slightly in that direction. As a result, they are then further from, and hence less affected by, the bottom-right GPMP. On each frame they are pulled slightly more in one direction, meaning that the effect gradually increases until it plateaus and the water stops moving.
At a guess, it's a particle order thing. On each frame, the 'actions' of each particle are processed in sequence based on the ID numbers of the particles. Since particles are 'naturally' numbered in ascending order from top-left to bottom-right, this means that some things that should be simultaneous, actually aren't.
No, i mean why is it all in one place?