[reneherse]

The nick you're describing creates what structural engineers call a 'stress riser' in the material of the bag. It focuses the force you exert on the bag to a small point, from which the tear will originate.

Stress risers are a common cause of structural failure, and are usually designed around. Some well known examples where they cause failures: rectangular window openings in aluminum fuselages, knots in fishing line, a poorly designed head tube lug on a steel bicycle frame.

In this case, however, the failure of the material results in a good outcome: snack time.