It sounds like this problem is similar to some of the potential problems encountered in carving/whittling; a certain amount of force is necessary to push a sharp edge through a section of wood, but there is a risk that the resistance to that force may abruptly and unexpectedly release.
The most common response to this is ‘always cut away from yourself’, so if it happens, the sharp thing only encounters open space, but in some techniques, the problem is overcome by bracing in such a way that an uncontrolled release of force will cause part of your body (say your wrist) to ground out against another part of your body (say, your abdomen), before the sharp thing in your hand goes anywhere bad. Demonstrated in this video around the 9 minute mark: Spoon Carving: Tools, Techniques and Tips for Carving a Spoon - YouTube
In application to the problem described in this thread, you can sort of achieve the same thing, either by bracing your wrists against your abdomen as you pull apart the bag, or by pivoting your knuckles against one another almost as if they are meshing gears - this permits more controlled application of force than simply pulling two points apart from each other in open air.
This seems to be analogous to the situation with traditional glass bottled ketchup. You tap-tap-tap and then whoosh, out comes a whole lot.
The reason for this is that ketchup is a non-Newtonian fluid. I.e., it has more than one flow rate.
Perhaps there’s a “non-Newtonian” equivalent for the materials bags are made of. More than one tear rate. Tears slowly with some effort then quickly starts tearing with virtually no effort.
A thousand times this. Because when they “propagate all the way down the bag, making it useless”, you then have to waste a large baggie to keep them fresh. Scissors, yeah.