I recently moved, and during the process I stored some things (canned soup for one) in a non temperature-controlled storage unit.  As you know this winter has been science fiction cold, and the cans froze.  A while after the move, I decided to have some soup.  It had been more than enough time for it to thaw, and the can wasn't cold.  However when I dumped it out, the consistency was was ... well ... wrong.  It was sort of like refried beans - but imagine that the refried beans had been left out for awhile and they had dry patches throughout, and then imagine somebody mixed in cottage cheese and sour milk and then blended it with maggots.  It was kind of like that.    There was no danger of me eating it, and I didn't connect to the freezing immediately, but it has happened again.  Can anyone explain, or bare witness to, this horrifying soup behavior. 
Your soup (and almost every other canned food) is mostly water. Water expands when it freezes.   This exerts a fairly large amount of force on the can, which will at minimum cause it to bulge (while frozen). It also potentially causes the can to rupture, possibly only a tiny amount at one of the seals.   If you were to transfer the cans to a freezer at this point, they'd remain safe, as freezing would prevent spoilage. Of course, you have no way to know for sure that the bulging is from freezing only, and not from microbes.   When it thaws, the water returns to its original size. This creates a vacuum inside the can again, at least if it didn't burst. That can also un-bulge the can. But any microbes that got in remain, and can begin spoiling (or worse) the contents.   But at that point, there may already be microorganisms allowed in when one of the seals gave temporarily under the pressure.   It may be possible to carefully inspect the seals to see if they've been damaged. It's also probably not worth the risk, especially for low-acid foods.  