Chance of ricochet is always there, and I've found that .22 is one of the worst. Problem is it's low velocity makes for "bouncing back". This, most of the time, is created by a flat surface that is perpendicular to the shooter and not by something curved or at an angle to the shooter. If the junk articles were responsible for the "bounce back" directly towards the shooter and were not readily seen behind the targets, then the bullet would have had to "bounce back" thru the target backer. Having seen .22 "bounce back" because of rocks in the berm, I'd be just as suspicious of them. I've also seen .22 "bounce back" when hitting bolts or large headed nails sometimes used in target stands, especially those that allow some movement when hit.
I'm not saying the "junk" was not responsible for the "bounce back", just that I'd consider all the possibilities before coming to a conclusion. Any thing hard in the berm is a hazard and folks depositing junk in one is foolish even if doing harm(causing trouble) is not the reason. I'd talk to the landowner and make sure he knows about the junk and see if he may know who's responsible. Odds are it's someone he knows that just didn't know any better. I don't use anything flat in my target stands anymore because of "bounce back", not only from .22, but from low velocity handgun ammo. I use round posts and pipe instead of 4x4s and angle iron/steel. Plates are always hung at an angle that direst ricochets to the ground. I use the least amount of fasteners I can when attaching plywood to posts and use screws with small heads instead of lags and spikes. I also insist everyone use eye protection....everytime. Even when one makes the best attempt to eliminate ricochets and "bounce back", shoot enough and it will still happen. Glad to hear no one was hurt.