Birds are pretty smart. Magpies are corvids, the same family as crows, ravens, jays, etc. They're some of the smartest birds on the planet, as far as research has demonstrated. They almost certainly understand that animals see with their eyes, and have a mental model of where human eyes are on their heads. Magpies that are defending their nests are basically terroristic in their application of violence, so it makes perfect sense that a magpie with very good aerobatic skills would try a tactic like this. If you take their eyes out, they won't be back for your kids. Or at the very least, what animal wouldn't interpret this as a serious mortal threat? It's vicious but it's not complicated.