Wheel has got it, most likely
The shell stops are the Achilles Heel of an 870 in my opinion and can fail if the gun is stored loaded for prolonged periods (as in a duty gun), shot regularly with high recoil ammo (slugs and buck), is stripped regularly (trigger group) or was built on Friday.
Typically the darn things will fall out if loose when the trigger group is pulled.
A common failure at our academy when I was there on temp duty. Guns were shot 5 days a week, buck and slugs by trainees.