Grab one of those jugs from the garbage, fill it up with water, squeeze it. See where the water comes from, from there maybe you can figure out why it's happening.

I've had a milk jug split on the bottom along one of the "seems" or ridges after slamming it down on the counter a little too excitedly. Maybe you're used to the force-absorbency of a soft bag and transporting/ handling it in a way that's breaking it?