I'm not quite sure how to word this, however as the new starfield is made up of 3 vanilla starfields rotated and tinted, they sometimes end up converging. The stars eventually line up in rows of three. These rows compress down to a single point, then expand back out into a row of stars. This takes place over a few seconds and eventually the rows break up. This can be reproduced simply by opening a world with the new skybox and waiting.

Latest Forge and Botania. No optifine or stuff like that.
Pics (was in a bit of a rush, sorry for ugliness):

Log (nothing too interesting and I was playing for a while so this is GIANT)

Personally, I think it looks pretty cool, and it might not be easily prevented in the first place, anyway.

This should be pretty easy to fix, I expect it's just randomizing the orientation of the starfields, but I never EVER want to touch Minecraft's rendering code.