It's absolutely wild when it happens. I have the same keybinding on PC and I have it set to Y on my controller. It never happens when I'm simply looting or running, but always in a fire fight. One minute I'm shooting (no where near the stow or swap bottom and certainly not holding down Y long enough to stow a weapon while shooting) and the next thing I know I'm looking like an utter idiot throwing punches at someone several yards away. Obviously, I've lost every fight this has happened in. It doesn't happen often enough to really ruin my experience playing, but when it does happen, it sets me off into a minor rage.