I get this too. My guess is that usually it happens at low speeds, coasting, while slowing down, so there is compression slowing happening and there is tension that builds in the drivetrain and it is all released at once and the clutch somehow doesn't want to disengage so it sticks a little. It happens to me when I am coasting in 2nd gear slowing me a bit downhill, or 1st coasting as well, usually the RPM's high enough to have the effect of slowing the car.