As the Earth rotates on its axis, the planet flattens slightly at the poles and bulges at the Equator. This dynamic causes the planet to wobble slightly.
As threads get longer, they take longer to travel through the internet and therefore as more subject the the effects of Earth wobble. Just an ease of use thing for the boards, as shorter threads are more stable.
I have no f***ing idea why 1,000 posts is the cut-0ff.