I took off my backpack and my items fell out. Why?

The storage section belongs to the backpack itself, so when you take the backpack off, its contents have nowhere to stay — they drop on the ground at your feet as a single bag. This happens even if your inventory has free space.

Pick the bag up right away. Anyone can loot it while it's lying there, and it disappears once it's empty. You can only take items out of it, not put them back in.