KT 150 failure

I bought a set of 150’s early this year for my Primaluna Dialogue Premium.  I’ve always run Gold Lion KT 66’s with no problems. Last week, during silence between songs, I heard this soft fluttering sound thru the woofer.  Cornwall’s.  The slightest tapping of the floor in front of the speaker produced the sound.  I went to the amp which is @6 feet away, and all looked fine- no red tube fail lights.  But the lightest touch to the far left tube really made the woofer flutter.  No red plate, static or popping.  After cool down and restart, tube is quiet until it warms up, then same situation.  Pulled them all and replaced with KT66’s.  All good now.  But I have a geeky question.  What happened with that tube?  In Dynaco Days, I had more dramatic failures.  It’s a fail, but what failed?  Thanks in advance for satisfying my curiosity.