Starter keeps cranking

On starting the car this morning (Camry 2000), it started nicely as usual
but the starter kept cranking. Nothing to stop it. Removing the key stopped
the engine but the starter kept cranking. Nothing to stop it until I
disconnected the ground terminal of the battery. Without the key in the
ignition, reconnecting the battery terminal resulted in the starter cranking
again. I know the solenoid got stuck but the problem was so sudden that I
wonder what exactly could be the cause of this reaction.
J.G.