Unacceptable!

Maybe overbooking or family and friends on standby - your seat may have been re-assigned anticpating you would not make it. I was once prematurely offloaded from a Swiss flight and could not board despite being at the gate when it still said "Boarding" on the screen and it was 13min prior to the planned departure. Lost half a day as well, sadly one of the last half days my beloved grandmother.
No compensation.