Waiting our turn in line, and my printed ticket sounds an alarm when gate agent scans my ticket. Nothing is said while a document is printed and handed to me. The line was long and the plane was full and I thanked the agent. Upon boarding the airplane we found another passenger sitting in “my assigned seat”. That’s when we looked at the additional document the gate agent printed. It was a SEAT CHANGE from the aisle seat to a middle seat. The flight attendants on-board where extremely helpful and knowledgeable. They contacted the gate agent, etc. but I was stuck in a middle seat for 6 hours. I contacted Alaska Airlines when we got home and they said “Sorry, here’s a $25 credit towards your next flight”. We never received an explanation why I was bumped from the aisle seat. Maybe the gate agent was afraid of explaining the seat change document, as she swiftly reached for the next passenger’s ticket, and left the explaining to the harried on-board flight crew.