First class reassignment?

First off to be very clear I’m NOT complaining, just curious.

Had a first class flight, Pensacola to Orlando via ATL. Changed the 2nd leg of the flight to an hour earlier (same day change) and paid an additional fare difference.

But for some reason, while I kept a first class out of ATL my Pensacola flight - which did not change - I was given a new seat in the last row?

Wondering why i was kicked out of my original seat? I’m sure there’s some sort of operational reason but can’t think of one?