Not just Russian. There have also been a few cases recently where spent Centaur upper stages -- which were thought to have been completely passivated -- broke apart for unknown reasons. These were in a geosynchronous transfer orbit (with apogee near geosynchronous orbit, perigee down near LEO) so the debris won't decay any time soon.
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.
(May I ask what definition of ‘passivate’ you are applying here? Even allowing for flexibility in the definition or autocorrect, and applying context, I can’t fathom what is intended to be conveyed by ‘upper stages - which were thought to have been completely passivated’)
> "The incident is the fourth time a Centaur upper stage has broken up since 2018. In the earlier three cases, the breakups created hundreds of pieces of debris, but no known collisions. That has raised questions about a potential design flaw with the stage or a failure to “passivate” the stage by venting propellant tanks and draining batteries, removing energy that could cause a breakup."
