This seems to be a fairly common thing--there has been a thread or two on this here in the not so distant past, and I've seen a number of horses that "pop". We had one filly do it here this spring--she was having kind of a growth spurt, and the one day I was trotting her in hand & she was REALLY using herself. Next day she was popping--you could see and hear it. We trimmed her feet so the toes were a bit shorter, and gave her rest & some glucosamine supplement, and she quit popping and hasn't done it since.
Many people, even judges, seem unconcerned by popping hocks. I have a friend who was selling a couple horses with this--she pointed out to the buyers that the horses had this tendency, but the buyers weren't at all concerned about it. They wanted driving horses and didn't seem to think it was any problem.
In most cases it does seem that if you trim the toe back a little bit so that breakover is a little bit easier, the popping hocks quit popping.