Well it wasn't like the Hun's tactics were totally new, far from it. And the other steppe menaces to Europe, that ended up in Hungary, the Avars and the Magyars also did pretty well, but they were also pushed to the west by stronger groups.
The successes likely comes from issues with leadership and realitive strength on both sides, as well as the fact that nomadic tactics, despite not being new at all, are often hard to counter by settled folks, for multiple reasons. Even without the Roman weakness the Huns probably would have done well, but nomads are very opportunistic (for good reason).