Damping keys in any additive animations will make physics for the bone freak out.
.Keyed values don't seem to matter
.The longer the animation is active the longer it'll take to return to normal after being deactivated (damping feedback loop, going into negatives perhaps?)
To reproduce:
-Open Spineboy
-Give physics to the gun
-Set an animation with a damping key for the gun's physics (it doesn't matter if it's 0, setup, or any number)
-Play test animation as additive on playback
(ignore the animation name, it's not inertia)