Well, no apparent drama but often in these cases it's not a question of an immediate failure but over time a problem building up. Excess heat is the best way to ruin a tyre and exposure to a hot exhaust is just that. What happens is that the tyre's oils are dried up and that reduces its flexibility.
Spares mounted on the rear door also have this problem through constant exposure to harsh sunlight and not being used, which is why wheel covers are useful. I must admit I don't use them but I go through tyres quickly enough that I don't have the same spare on my door for years on end.
