Lemmings are bat-shit crazy.

And before the Bleeding Hearts Club pipe up with something like, 'WHY ARE THEY HARASSING THAT POOR CREATURE!?', from all accounts on the innernet (a place devoid of lies), Lemmings are known to be very aggressive animals, and it's not uncommon for them to actually run up on skiers in Norway and attack them. Which is hilarious.

From Wikipedia:

"Lemming behavior and appearance is markedly different from many other rodents, who are inconspicuously colored and try to conceal themselves from their predators. Lemmings, on the contrary, are conspicuously colored, and behave aggressively towards predators and even human observers."

Still, he probably didn't need to chase it down the slope like that.