Oh Give Me a Break!

So I walk outside to throw my garbage in the dumpster.

I see a smallish bulging bag of garbage dangling from the lid of the dumpster. Clearly, when the garbage truck emptied the dumpster this morning, the bag got snagged and left behind. I'd noticed it earlier on the way out on some errands but didn't go out of my way to address it. Surely someone will recognize their brand of garbage bag and handle it. But they hadn't for whatever reason (probably because of the very sensible reason that they have no idea what their own garbage looks like, I imagine).

So, being a good neighbor, I grab the top of the bag of garbage and then lift the lid so I can throw it in the dumpster and hope that round two next week works out better. Simple, huh?

Of course not.

I'm holding the bag securely. I lift the lid which is holding on to a tiny smidgen of bag. And when I do that, the contents of the bag empty out on to the parking lot in front of the dumpster.

The bag of garbage, you see, was dangling open side down.

So yeah, I then picked up the loose garbage of someone else and deposited it in the dumpster.

Now, this isn't a "no good deed goes unpunished" sort of post. Although it darned well could be were I in that mood. (And since I paid my property taxes earlier today, why am I not in that mood?) No, this isn't a cosmic justice question. This is a science question.

What I want to know is how could that bag have possibly hung like that upside down all afternoon with only a tiny portion of the bag's bottom pinned inside the dumpster lid and not already have emptied out on to the ground?

What act of physics was triggered by me lifting the lid, which relieved the tiny focus of pressure on that tiny corner of bag, and which apparently determined whether or not loose bits of garbage would be pulled by gravity on to the ground at my feet?

I'm putting this in my X Files for study. Something is seriously messed up in my time-space vortex. Or something.