To absorb spilled beer.
Hmmm... maybe they didn't understand what the yarn was for.
Maybe mice had eaten all the yarn away, so the strings could resonate freely. And maybe the clavinet was set up right next to a wall of guitar amps, and while the band really liked the clavinet solo in the middle of "crazy train", the guitar buzz was stimulating the clavinet strings causing an eerie ambient wash of chromatic diversity. Obviously, there was no other option but to attempt to dampen the resonance by adding foam to the interior of the clavinet.
Uh, yeah.