It happens only while you are not holding the forearm, does it not?
Holding the forearm is required, if there is no spring to keep tension on the forearm, as in the 870, with the bolt closed and fired. Friction is what keeps the action bar lock from being actuated after firing, and that friction comes from the shooter holding the forearm on these guns. The action bar lock contacts the action slide under the bolt. When you hold the forearm, while shooting, it produces a friction from the recoil between the lock and the slide. When you start to pump the gun, the friction is released. During firing, the hammer falling puts tension on the action bar lock, via a spring, which will make it drop down from the action slide, so the gun can be pumped, after the rearward forearm pressure is released.
Winchester Model 12's, and the Mossberg pumps, use a spring on the magazine tube, so one can fire them without holding the forearm. Remington 870's don't have this.
If a Mossberg does this, something is wrong.