A friend of mine bought the 90" wide, 3 yards, for a quilt back and decided to rip off the excess width, she only needed 54". I told her to rip it rather that cut because it was easier to rip down a 3 yard piece than cut it.
It did not tear straight. She took it back to the store to show them what happened and was told that the wider fabrics were not made to rip, they will tear wrong because they don't have a good threat count.
This is the first time I heard of a thread count causing a bad rip. I have ripped muslin(not an expensive fabric) both with the selvage and against the selvage and it rips along the grain.