I had a puppy once who must have been Republican -- he ate his own poop.

1) Dog chases tail every once in a while on a lark, or because it's kinda fun, or there's something that itches.
2) People see dog chasing tail, think it's funny, give dog attention.
3) Behavior is reinforced, dog chases tail more, people give more attention, rinse, repeat.

Where's my science grant?

/ probably doesn't explain it all, but that's my theory for most of it

For the dog that chases its tail will be dizzy
These are clapping dogs, rhythmic dogs
Harmonic dogs, house dogs, street dogs
Dog of the world unite
Dancin' dogs
Yeah
Countin' dogs, funky dogs
Nasty dogs (Dog)

Atomic dog
Atomic dog