I'm actually not a native speaker either (I'm Finnish).

I think 八つ当たり might derive from the same idea as 八方 which means "every direction" even though it's literally "eight directions". I'm guessing the idea is that you're hitting things all over the place, even if it's not a suitable target.

EDIT: oh, I forgot to add that 八つ当たり is very common, so you definitely should look out for it to get an idea of how and when to use it.