GAs sometimes come onboard for a count. In this case, it was probably because the mother asked them about the carseat and they were on a power trip, as the FA told me they were.
I've had it with AA lga-dfw gate agent said no, then head FA said sure, we were the last row and we had empty middle seat, as I was making the way down the aisle I heard screaming, hey stop right now and the gate agent made me take out the baby and she took off the doona, and at DFW the gate agent and the FA had no issue to take up my doona they were just so much friendlier then the NY agent