Whenever you get a fine, it tells you why at the top right of your screen immediately after you get it, but if you're busy with something like nav panel, it's easy to miss it. It doesn't show it for very long. 400cr fine would probably be for something like bumping into another ship while speeding in the no fire zone coming out of the station. There's a 100 km/h speed limit that nobody takes any notice of. There's no penalty unless you bump someone.
Yep. I know.
The small fine came from leaving the station.
Turned out there was a bit of a firefight going on outside.
The muppet NPC in front of me either decided to stop in the mail slot to watch the fireworks, or was too scared to go out.
Either way, I got stuck in the slot as well.
Got the notice about loitering etc.
The big fine, I think I may have been scanned, but no alerts came up about fines.
Definitely must've been scanned with something illegal in your hold. Big fines like that typically only happen when you get scanned with a bunch of naughty cargo.

Saying nothing!
I've been scanned with illicit cargo before.
Never ended up with a fine that large.
Oh well.....