Why are my horses clumping together in one spot?
This might be a sub-problem of the bug MC-21109. The report says:
Mobs (the creeper) get stuck in northwest corners
This is a combination of things. For one thing, entities begin pathfinding from their northwest corner instead of their center. […]
This might also mean that they prefer the North-West direction, but I'm not sure. To be safe, I created a new bug report: MC-140545
For now, it just says what we figured out here, more testing would be required to find the general cause. But for now, it's reported at least, these tests take a long time.