Check overpair, bet top pair on the river?

An interesting spot I encountered recently had the solver checking most overpairs on the river while betting most top pairs.
Here is what the spot was:
Board: Ts6h2d8cQs (BTN vs BB SRP, 100bb)
Action:
FLOP: BB check, BTN overbet, BB call
TURN: BB check, BTN overbet, BB call
RIVER: BB check, BTN?
In this spot, the solver checks all AA and some KK and bets almost all of his Qx top pairs. Why is that?