At the end of the day, seat assignments are NEVER guaranteed -- even at the airport. Usually, it works out properly. Sometimes, it doesn't.

Some GA's look out for fellow employees. Others are rude and nasty to fellow employees (can't figure it out but it definitely happens). Sometimes GA's can easily explain everything that is going on. Sometimes the complete truth would just escalate the situation.

In the case of the OP, it is likely that a top level FF passenger (Platinum or higher) was on standby for the flight and didn't have a seat assignment. Perhaps the agent recognized the passenger from previous trips. The GA decided to "unseat" the OP who has NO status and reseat the the top level FF passenger there. If this happened, there is no easy way to "explain" truthfully what happened, except that the OP got trumphed by a top level passenger (not easy for the gate agent to explain without escalating the situation).