I'm regularly seeing tweets from the Piccadilly Line that say that there's
a shortage of available trains. Is this some newly discovered technical
problem, or the result of union action? This fleet is regularly reported
as LU's most reliable.

Here's an example from this morning: "Apologies for the gaps in the
service, this is due to lack of available trains. Our staff are working
hard to resolve this issue."

The gaps in service are causing overcrowding at busy stations, leading to
tweets like this, "Kings Cross station - Closed to prevent overcrowding.
Update to follow."

It's not just today -- it seems to be happening regularly.

These 40+ year old trains are expected to soldier on for the best part of a
decade before the NTfLs replace them, so I hope the problem is temporary.