When booking through a TA, they actually are requesting a specific seat rather than guaranteeing you a specific seat. That request may come back confirmed or unconfirmed at any point within 24 hours after booking. What tends to happen is that folks will request a specific seat (ex: 7C) that either is not available or they are not qualified to take ahead of time (i.e. preferred seat requested by non-elite) and they are not notified by their TA that the seat was not confirmed. No other seat is requested by the TA - presumably because the passenger did not know they needed to go back online and request an available seat either through the TA or on alaskaair.com - so the passenger is seated in what is left when the flight goes to the check in window. When they check in online or at the airport they suddenly find themselves in a different seat than they thought they had and believe that the airline "changed" their seat without realizing that they never had that seat to begin with.

Generally the only time Alaska changes the seat - barring equipment swaps and FAMs - is when someone books a designated disabled seat and it is needed for a disabled passenger or the airport discovers that one of the passengers in an exit row seat is not qualified to sit there. If the seat swap happens before travel and there is a phone/email in the record (not common in TA bookings) then an attempt will be made to contact the customer to let them know.

As a Gold75K, you would rarely experience this situation unless you routinely choose row 1/6 or the designated disabled seating further back - and even then it would not be common. If that happened you would certainly receive a phone call or email to let you know.