Almost involved in 3 car pileup

So my bud bought a 650 dollar 95 explorer yesterday and after about 5 miles down the road he hits a bump and i see the reverse lights come on and then for about 100 yards it slides sideways and then does a 180 and smacks into the guard rail gently...

The truck is in really good shape. Just has a tick from hell. I was directly behind him and in the other lane was a nice f350 with all chrome and a car trailer. Luckily we both stopped in time. but what the hell would cause it to go into reverse?

It doesnt have 4wd low and we cycled the 4wd to verify this right before the wreck.. could this have anything to do with this??