As Franck’s Prélude, Choral and Fugue moves through passage after fortissimo passage, the piano is moving, too—away from me. I’m already leaning forward to keep my hands on the keys.
Rolling the piano into position lines up the casters so they tend to move easily along the same line; in this case, the line along which my playing is pushing. And these casters have no locks. And the stagehands didn’t angle them, which would have prevented the motion. And I failed to notice the situation. The Bach Partita didn’t put enough energy into the piano to start it moving. But moving it is, thanks to Franck.
Can I pull the monster back? Manhandling a half-ton from a seated position isn’t easy, even if it is on big casters. Can I move the bench forward? Only with a risk of thunks and clunks. And anyway, from this point in the Franck, there will be few or no moments with hands free for such tasks.
It’s like one of those nightmares where you’re in an embarrassing or dangerous situation and somehow frozen, unable to get out of it.
P.S.: Everything came out OK. I even played the Franck well. Maybe I should play a “piano pas de deux” more often. 
