My flippant answer would be: they're crabby!
Sorry, I had to do it.
More seriously, I tend to think of them as fiercely territorial, even rabidly territorial, living in those little groups (herds? pods? tribes?) of theirs. Mostly because, in Skyrim at least, I've noticed them just going into warning stances like giants, wolves, and bandits if I get close enough for them to notice me. They attack when I get too close for their comfort, when I become a threat. The problem is: they tend to blend in to their surroundings well enough that I run into or over them before I realize it. 'S probably happened to all of us at one time or another.
There's also the Mudcrab Nation theory and the vast network of operatives they have, but that's too silly to be mentioned...