Flying (High) Reindeer

So that explains it!

From the Pharmaceutical Journal

Reindeer go to great lengths to search out the hallucinogenic fly agaric mushroom (Amanita muscaria) — the one with the white-spotted red cap that garden gnomes like to sit on. Eating the toadstool makes reindeer behave in a drunken fashion, running about aimlessly and making strange noises. Head-twitching is also common.

Fly agaric is found across the northern hemisphere and has long been used by mankind for its psychotropic properties. But its use can be dangerous because it also contains toxic substances. Reindeer seem to metabolise these toxic elements without harm.