Minor creatures frozen

[Bad Dog]

I sorta solved this problem, but I don't understand it and I thought maybe someone with more knowledge could explain what's going on.

 

I loaded up my Skyrim with everything in sight, using MO: SkyTest Immersive Animals, SexLab, SOS, Nude Creatures, Bestiality Extras, Creature Features, SexLab Romance, and SexLab Defeat. For a wonder it all worked. But I was so busy running around getting raped by every person and beast in sight that I didn't realize the minor creatures had stopped working. Mudcrabs, mammoths, elk, skeevers, were all frozen in place. They couldn't move or be hit. 

 

I figured maybe some vanilla animation had been stepped on somewhere along the way?

 

I tried using MO to turn off all the cruft, but no joy--took the whole above list out, and still frozen creatures. 

 

So I thought maybe it's a FNIS thing and I deleted and reinstalled the FNIS mod, making one mod of FNIS and another for FNIS creature animations. Ran FNISforUsers and Loot, and everything is working perfectly. (Well, mostly. Some animation jitters and so forth.)

 

So what's going on here? I don't really understand what FNIS is doing. Can anyone explain it? Or point me to a better explanation of FNIS? I haven't been able to find anything clear.