Upper stages breaking apart is kinda expected. It's a rocket, and it has rocket fuel (explosives) on board. Derelict spy satellites, on the other hand, shouldn't be just going kaboom for no reason. But the Russian ones do.
As noted by another poster, prior to the recent ASAT tests, one of the largest sources of junk was old Soviet nuclear-powered satellites leaking coolant and/or having their pressure vessels explode. Mind you these are actual nuclear reactors, not RTGs.
No, all residual fuel (and anything pressurized) should have been vented after the last burn and there shouldn't be anything left on board with enough energy to cause it to break apart.
As noted by another poster, prior to the recent ASAT tests, one of the largest sources of junk was old Soviet nuclear-powered satellites leaking coolant and/or having their pressure vessels explode. Mind you these are actual nuclear reactors, not RTGs.