Worm castings are moist and somewhat sticky. Due to the moisture and mild compaction at the bottom of the Urban Worm Bag, you will find that the contents of the Bag don't simply all fall out.
The Bag's conical design also means that the bottom is more narrow than the top. This causes some sideways compression called "bridging" which also prevents the entire contents of the Bag from falling out, assuming the Bag isn't nearly empty when you harvest.