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.
It's a real mystery.
And used in context (this context),
> "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."
https://spacenews.com/faa-to-complete-orbital-debris-upper-s...