930, tail pipe is on fire while start the engine.

[Burint]

My 930 is in good running condition.
I didn't use the car all day until later in the evening.

I started the engine, then there was a backfire but the engine won't started.
While I was still in the driver seat, through the side view mirror, I saw a foot long of flame came out from the tail pipe.
I jumped out from the seat to get the fire extinguisher.

There was a small amount of fuel in the tail pipe that's still burning.
Luckily, the fire went off by itself.

The real scary part isn't about the burning pipe.
When my mechanic arrived a day later. The engine started up just fine.
He took the car out for an hour with no issue.

He came back and said he couldn't find out what causes my the pipe to spit fire.
Any idea what went wrong?
Many thanks