Well, other than you reversing into something it may be what happened to me, some other person has collided with your bumper damaging it, they may have had a quick look and thought "that,'ll be ok" and jogged off leaving you with said damage. This happened to me and I suspect the Transit twin wheel truck that was parked close by was the culprit, however, no evidence, no witnesses, no nothing. Now I have 5mm thick "discreet" bumpers that look almost as the originals but will certainly not surrender in my absence, but hopefully inflict a retributional dent on the offenders vehicle, so they have a nice reminder not to nudge my truck. They also serve to prevent damage when 'froading and are part of the spare wheel/ jerry can carrier. Whilst on holiday in Spain couple years back, had a chap try to push the bruiser while it parked ( legally) along the road so he or she could park in a ridiculously small gap, the bruiser never gave an inch, the bonnet on the other car did, a perfect impression of the tow ball in it, some people must have the brains of a chocolate mouse, with its head bitten off!!