Two thoughts:

First, there is a history of the seat change actions in the system. I'm not saying that anyone will actually tell you why you lost your seat, but there will be an entry in the history of the reservation that indicates who moved you and why.

Second, I am somewhat dumbfound by the fact that you chose to cancel your trip because they changed your seat assignment. I get that the one they moved you to was much worse than the one you originally had, but that seems like a crazy reason (craz-y reason??? ) to cancel a trip.