Pure speculation, mind you, but this might be part of a plan for assassinating Oyamada. Nobukatsu must've taken note when Reiri almost assassinated Ieyasu, and then climbed and fought in those steep hills/cliffs during the siege, followed by running up that slope. Perhaps Katsuyori intends to to bait Oyamada into capturing Reiri-as-Katsuyori in order to deliver him to Nobunaga as a hostage? Or does he intend to murder Oyamada and blame it on Nobunaga?