Instead of running around in a circle, you would think they would just run away........
Did you watch it? they were harder to kill this way AND they could kill the hunter.
Reindeer emerged as a distinct species about a million years ago (Yep, I had to look it up... I’m not a reindeer expert!). The cyclone defense probably worked great against their main enemies (bears, wolves, saber-tooth tigers(?)) for much of that time. When man came along, the reindeer just kept doing what worked well against all the other predators.
Ever see old Westerns and wagons?
Being herd animals, they like to follow each other. Just like the ancient plains Indians would get buffalo to stampede off a cliff, it is probably an accident the reindeer just happen to end up in an "infinite loop" following each other.
I bet there are many dead reindeer afterward from being trampled so the hunter has it easy.