Wreck related or not

It's definitely possible that it could be accident related. The jarring impact of a rear end collision can push the exhaust system around and potentially break the "honeycomb" inside of a catalytic converter, causing it to clog up and cause issues. Also, obviously, if that impact can break the honeycomb in the catalytic converter, it can easily break a worn out engine or transmission mount. If one mount gets broken, more stress is put on the other mounts, which are also already worn down, and can cause them to break if you keep driving it.

I would say that it's not terribly likely that both of the honeycombs broke inside of their catalytic converters. But, in a two converter exhaust system, many mechanics will want to replace both converters when one goes bad, because generally the other one isn't going to be too far behind. The same goes with mounts.

I hope that this information and opinion helps. Good luck with the mechanic. And remember, if you don't like the mechanic, you can always go to another one to get a second or third opinion.