Usually if an upper stage experiences a failure before it can "passivate" itself (empty tanks, drain batteries, etc.), it's only a matter of time before it explodes. For recent examples, see any of three Breeze-M upper stages (pictured below), including one that resulted in thousands of new debris pieces in relatively energetic orbits -- the U.S., of course, has also had its fair share of upper stage explosions, although not as many in recent decades.
My question is, what is (usually) the failure mechanism that leads to these explosions? Why is it (seemingly) inevitable?
Note: I am not asking what leads to the initial failure that strands the upper stage in the first place.
I suspect there is a common underlying reason for all of these events, but if this is considered too general, then please answer the question specifically for the recent Breeze-M mishaps.