What you experienced is a "Harp Explosion."  I have known it to happen
a few times.  Basically the glue joint between the pin block and the
back frame gives way with a real loud "BANG."  This is due to
unfavorable conditions under which the piano had been kept and/or bad
design.

It seems to happen more frequently to pianos with "partial plates" --
a harp plate that does not cover the pin block -- or the plate bolts
at the top are not threaded into the back posts, or the back posts are
soft pine.  Every Kimball player I have seen from around 1910 seems to
have this problem.  It's a partial plate and some of the pin block bolts
do not go into the back posts.

I had an early Hobart Cable piano in the shop once that in foreseeing
this problem, the top plate bolts go all the way though with flanged
nuts on the back, much like the "Star Bolts" you see on old buildings
to keep the walls stable.

Brian Thornton