When does a TPK occur?

[Yaztromo]

I think that splitting the party or anyway inner fights that prevent characters to focus on the enternal enemies are the most likely cause of TPK in my experience.
It can happen that there are multiple deaths against the BBEG, but rarely it's a TPK (at least somebody survives or run away).
The other frequent cause depend on the party in general not being on the same wave length of the game master, for example with characters beating up everybody and everything showing up in front of them when the adventure was all about investigation, or deciding to fight the obviously overpowered enemy when all the adventure was about finding a way around by outsmarting it, or similar...

 

[Yaztromo]

I just learned another method to cause a TPK.

Put a big red button labelled "Push this to cause TPK" in a dungeon. Watch as someone inevitably pushes it just to see what happens.

Sounds like a perfect feature for a Gamma World adventure...