I think it was more pure animal behavior at work. The little kid challenged him by standing before him beating is chest and the gorilla did what one would do in that situation. Similar to the lion one. Not a big deal to me.
But true too, zoo animals suffer boredom, frustration, etc., etc. Not a big fan of the zoo anymore.
People should be nicer. According to a movie I saw, those apes are going to take over the planet.