Chimps may have seen humans using spears. They are masters of mimickery. So long as existing chimps keep doing it, the practice should survive.
Maybe, but the spear has not been a weapon in that region of Africa for some time, and the chimps have just started using spears (or so it seems).
Methinks it's even more simple -- chimps have long used pointed sticks to spear termite mounds to eat termites. The fact the chimps hunting with spears jab the spear into the brush or a hole in the tree and then lick the stick seems to be an outgrowth of the termite mound maneuver.