What causes a 'flame out' on overrun?

[twin plugged targa]

Newbie question here.
We all love to see it, when a 930 spits flames out the exhaust.
I assume it is excess fuel, but why?
Engine running very rich?
Do all 930's do it?
Do stock 930's do it or only modified engines?
Thanks
Ben

[Tilikum Turbo]

934's do it...935's do it.

My guess it's a modified 930 with the turbo reconfigured to belch flames out back.
(dumping unburned fuel/air into the atmosphere, ignited by the hot exhaust)

[A930Rocket]

My 930 with a 1 bar spring used to spit a huge flame out the B&B muffler. I could see it in my rmy mirrors, but over time and mods, it doesn't any more. Not sure why..