It's interesting that the outer casing broke away rather than the spur breaking off at the leg join. I've had a few roosters and hens break off spurs due to fighting or getting struck badly on a rock or brick and they invariably break at the leg.
Those roosters that have shed cases shed without trauma as far as I've been able to tell. Maybe a slight tap in the cases of Henry and Twitch who I ma fairly certain weren't involved in any fighting at the time.
So, it's reasnable to assume that there is a point at which the case will come off rather than breaking the whole spur from the leg. It would be handy to know if there are exterior signs of imminent case shed.