I watched Nimbus 7 make a pass over a week ago, and it seemed to
brighten and fade, then brighten again before it entered Earth's
shadow. I wasn't able to spot it again until just a few minutes
ago. It brightened and dimmed twice over the space of about seventy
five seconds (transit time from alpha Andromedae to alpha
Cassiopeiae). Brighteness varied from about magnitude 6 to 8.

I've spotted Nimbus satellites before and never noticed this variation.
Maybe a fuel tank or battery has gone "bang."

--Bill Thompson