It may be hunting for the next split point.
One thing that has help me with a problem (finding a) split point, is to temporarily select a 500K (was 260K) Break files size, for the first split pass.
Once that completes, do a EPUB to EPUB with the 260K value restored.
That has worked out 98% of the time. The other 2% I attacked the $%^& Monster sized <div's> coding with Sigil