Alot of older people can't hear clearly with back ground noise present.You perceive in typical calm retrospect, in the heat of the fracas everyone's senses are working perfectly, the cops yelling in perfect unison [not normally] over the din of traffic of the bus stop area, vehicle noise, etc; and the gentleman, who to his surprised dismay has suddenly found himself losing his balance, finds his firearm has fallen out of its holster and is lying on the ground in the middle of an altercation between others has focused on only one thought - get may firearm back in hand to keep altercation members from getting hold of it.
LEs believe all team members have to scream commands and those being screamed at can understand and heed those commands. Watch children at play react then have a group of adults hollar the same command at them, see what chaos ensues.