The best existing phrase I have is, “Dropping your guard.”

It doesn’t necessarily imply anything about why you dropped your guard, and only slightly implies that the bad thing happened while your guard was down.

We could coin a new phrase here by brainstorming. I’ll start:

Fumbled the shield

I think shared ownership goes against human nature

I think the vast majority of things are shared. Air, water, soil… roads, schools… your education… the bathrooms at the supermarket… the murals/sculptures that make your town look less like a shithole…

It’s perfectly in line with human nature, actually, to share things. People who amass hoards of wealth and fight for power and control… those are the weirdos.