Explain Like I'm 5 (ELI5)

I've had a couple of articles come up for me like Darwin award winners walking into helium balloons while they're still being filled at ground level, or people walking into buildings (is it far underground?) and just asphyxiating before they realise.

How do these conditions come about? What about cave diving? Where do you get areas open to the environment where oxygen doesn't reach? I get with helium balloons the gas is being forcefully displaced. How else do you encounter oxygen absent scenarios?