Brown must have been acting butt-hurt or pouty after getting benched in St. Louis. Payton probably waited to see how he reacted in practice after the dust settled, and hopes that the fire would be lit. Brown most likely let him down and was sulking, so the switch to a small school, raw as all get-out rookie gets the nod.
Even if Armstead holds his own, it's not going to help the defense's road woes in stopping the run and getting pressure on the QB.