They probably do that because the AI is programmed to be really concerned about you. If they share a border they are likely to beef it up. I've had the same thing happen with the Byzantines. I was HRE once and I was at war with the spanish and even though I had never attacked a province of theirs they piled -every- unit they had into the two battle neck provinces (aragon and naverre?) leaving their southern provinces with zilch while they were at war with the alhomods. That was the end of them needless to say.

I've had the Danes grow pretty big but that is very seldom, as they usually spiral into debt. I've never seen the aragonese get past 4 provinces. The patriarch will sometimes take most of Italy but not much beyond that. _Aetius_'s game is an incredible exception. I think the Pope goes apes*** after reemergences in comparison to his docile self beforehand. In my longer games the HRE rarely lives too long. The novogardians seem to do well until about 1230 and usually are nothing to speak of after 1250 unless I start a late game.