[NEFARIOUS]

The only theory I have is thanks to them manhandling your packages (pun kinda intended), it creates a slit in the tape (the kind of tape that you have, not the kinds with little fibers in them although the glue for them is garbage sometimes, but that's another story). If pressure is applied to that slit, it tears itself open... In fact, my lazy way of opening boxes is to cut the ends of the tape off, make a slit in the middle, punch it to expand it, then tear it open.