[EDIT] Found it!
"They did a temporal noise reduction pass, so it compared frames before and after to try and identify dirt and other crap - in scenes with not a lot of contrast differential it struggled, and copied parts of previous and future frames into the current frame - unfortunately it was also iterative, so the frame got processed and the processed frame was then used for the next frame compare - resulting in those gorram trails."
The SC's weren't mastered by THX, so while there is more "noise", there are fewer added video artifacts as well. As such, with the computer resources and software at our disposal, it would make more sense to do noise removal on the SCs (i.e. TooT), rather than risk more complicated artifact removal on the THX LDs.
As I said before, the X0 Project knows far too much about these LDs.
But this is an excellent example of why the project is taking so long, and expected to take even longer - there are a lot of technical decisions to be considered and made before actual video correction can be started in earnest.