Stealing. Popping off tags of higher price merchandise and switching for clearance or lower price tags. Thus the references to heavy bags, because the items are actually purchased and bagged at the lower prices. Possibly also removing sensor or ink tags when shoplifting.
Macklemore and Ryan Lewis in Thrift Shop say "I'm gonna pop some tags, only got $20 in my pocket. I'm huntin', lookin' for a comma (4 digit prices)...." "Manuel, he's poppin' tags."