What are we looking at?
I think what happens is that the knot slips on the line, but not the sleeving which results in the bunching effect.
This happens to all my 50 pound lines whether I tie them or if they are bought professionally done (as in this case).
I've also seen 90# do it, but much less prevelant.
So, I now tie a loop of bridle and larks head onto to that for my 'sleeves'. Never bunches and I've not yet had a break either.