Why does my cursor sometimes get stuck as the default pointer (black arrow) in Safari?

This only happens in Safari and I am on 13-inch MacBook Air. Sometimes, my cursor just doesn't change from the default black pointer when it is supposed to (e.g. when I hover over a link). Note that it initially works fine; suddenly, then, it just doesn't change after a bit...

Why does this happen and is there a way for me to fix it?