This happens because their title is titular, and because all of the surrounding realms have de jure claims on land they hold. This means they basically get dogpiled, and even if they win the first few wars, their troops will be low enough that they'll start loosing. Once they've lost the first war, that starts their death spiral since the surrounding realms still have claims, and now the Magyars are out of troops. I believe the Magyars have a titular title because they were designed for the TOG start, where they have massive event troops to discourage the surrounding lands from jumping on them, and almost immediately start in on creating Hungary. Here, though, they don't have that, so they get stuck.
Oh, and their culture doesn't match their land, so there's also a risk of revolts. (Are the Magyars Hungarian in the TOG start? I can't recall.)
