For those who have old fashioned MIDI cables in your setup, I did two more shows with a new MIDI Out cable, and I had no issues.

John Fast at MIDI Solutions confirmed via email that a bad cable could be responsible for the unwanted behavior. Instead of sustain on pedal down, I had sustain on pedal up, a portamento effect added to my voices, and the keyboard jumping to the wrong registration. He said that if the cable shorts out in the middle of a MIDI command, the keyboard might receive just a fragment of the command. Maybe even two fragments could get spliced together.