The wiki article seems to list three possible causes for the attack:

Travis was on medication for Lyme disease and was also given tea laced with Xanax before the attack. (Apparently the Xanax was prescribed by a Vet… though no source is listed for that exact claim, it showed up in the autopsy and may have been administered regularly?)

The woman who was attacked had a different haircut and car than Travis last saw her with, and this may have confused/enraged Travis.

A Tickle Me Elmo toy (Travis’ favorite toy) being presented by the victim sent Travis into a rage.

…

In conclusion, chimpanzees should never be pets, and you especially should not give them benzos.