It might have been imagination, but the chimps in the movie seemed to hesitate when their victims were standing (or getting) back on their feet. Apparently, the size of standing humans is an intimidating factor. They should never have lowered themselves to the size of chimps by sitting amongst them.
Some basic knowledge of animal psychology is indeed useful. Assuming a threatening posture in a situation where the animal can't escape or back off wouldn't be the best idea
Remi Vredeveldt
"Hysterical knowledge is often mistaken for historical knowledge"
Boni enim duces non aperto proelio, in quo est commune periculum, sed ex occulto semper adtemptant Vegetius Liber III, 9:5