I've noticed that there's a pattern of movement: they appear to be separating on the Z axis. The only reason they're also moving left is because I put in a wind force field, but this unexpected effect still happened before the wind.
It looks like your particles are getting a little normal velocity (i.e. being "pushed" off the emitter's surface). You need to turn down the Normal value under the Velocity rollout of the particle settings.
Set this value to
0.000 and your particles shouldn't move.