Getting crazy because of the intense radiation underground,
the wurms surfaced above the ground.
So this is why the wurms don't have Burrow ability? Because the intense underground radiation makes them don't want to go back there anymore?
Correct, and they end up destroying houses and farmland when they surfaced
kinda like how discarding opponent handcards is.
Bear in mind that a wurm isn't a giant underground worm, but a wingless dragon. (Well, okay it's both but still e.e)
You're thinking of Wyrm. Wurms are indeed large, often underground worms.
http://gatherer.wizards.com/Handlers/Image.ashx?multiverseid=262673&type=card
I rarely sees people mentioned wyrm,
my Earth Wurms are referring to underground worms which are also rarely mentioned.