Time bomb? I don't know.
I think it will depend a lot on what's getting traded in and just how much. For used auto dealers, this could suck big time in a few years. The same goes for those who buy used parts or can only really afford a used car. In a few years what do those people do?
I think time will tell if we have another credit bust on those one, allbeit a smaller scale. If you have a vehicle with no payments and trade it in under cash for clunkers, you now have a payment and likely higher insurance.
Not nearly as predictable as the housing market crash was though, and certainly nto to the same scale if it does happen.