Can't find proper information.
I read somewhere that it's caused by excess fuel in the combustion chamber, and somewhere else that the ignition partly happens in the intake (similar to a backfire).

Some explanation would be much appreciated.

Thank you.

Tailpipe fire Causes

With engine running

too much fuel and not enough air.

Mostly due to insuffcient compressor output for a myriad of reasons

With engine in start sequence

Pooled fuel dribbling out of the burner and eventually being ignited during the start with some of the fuel behind the burner section.

Oil leakage from a bearing compartment in the turbine collecting at the bottom of the engine and migrating aft to the tailpipe.

More importantly is what action to take in-the-event of.

They can occur after shutdown too, if the HP valve doesn't close fully. This allows fuel to dribble through the valve and into the still hot combustion chamber.

Makes for a great fireball!