A pretty long time ago I and my friend were building a giant basketball court, and out of nowhere a big explosion happened and there was then a big crater in the ground. We had absolutely no mods at all and no one could have done it, because we were not playing online (we were playing split-screen on the Bedrock PS3 edition).
I don't know if this was a glitch in the mapmaking, because it was a flat world. I am just wondering.
Best Answer
There are no meteors in Minecraft. The closest thing would be a lightning strike during a thunderstorm, but it would only set things on fire, it wouldn't create a crater.
An explosion can be caused by a few things:
If you were playing in peaceful mode, we can ignore the mobs, which only leaves the explosive items. None of those would explode on their own though. Maybe your friend was pranking you?
If it was a glitch in the world creation (very doubtful in a flat map), it would have been there forever, not appeared suddenly with an explosion.
Edit: seeing that your custom flat world was made of wood planks, yes, a lightning strike could have caused it, if your world has the "fire spreads" option enabled, which I believe is on by default. However, it could only have hapened during a thunderstorm. Was it raining? Did you see lightning? Usually you get multiple strikes during a storm too, which would mean more than one explosion and crater.
It's also worth noticing @SF's comment: TNT, end crystals and explosive mobs don't cause fires, so the only real option is either the lightning, or a charged respawn anchor.