Overload built-in Onan gen. without popping a breaker?

I was testing my gas-powered Onan 5500 the other day. I was running both air conditioners and the water heater. It seemed to be handling the load well but after 20 minutes, it just stopped - no sputtering or anything - just spun down suddenly.

I was able to restart it and it worked OK but I'm wondering what would cause it to stop like that. There's plenty of oil (in fact maybe a bit too much oil).

No breakers were popped.