brute force method at supermarket

A lot of the times when I was at the perishables section of the supermarket, I saw some shoppers (especially the males) trying to pull the perforated HDPE plastic bag with a forceful jerk perpendicular to the dotted line. I don't know if they were trying to show how macho they were, or they were plain toopid, but all one needs to do is to just gently tear along the pre-cut (aka perforation) and you get the plastic bag off from the roll. When they try to force the plastic bag loose, sometimes they damage the roll holder, or they end up hitting other shoppers.

It's got something to do with mechanics of material or material science, which I have conveniently forgotten most of the stuffs. Crack propagation?

Or just try breaking a piece of newspaper into two by 1) pulling it on both ends; or 2) tearing it like how you tear paper. Just see which method uses less force.