I believe that it is due to your high level.
No it's not
It's due to the guy you last fought leaving the game, giving you the experience needed to level up. Else there's another reason too it I'm unsure of. However it happened to me around lvl 40 and when I posted something like you did on the forums people thought I was lucky I'd managed to go 40ish levels without coming across this bug
To make sure it doesn't happen again, as you're about to level up play single player for the experience and you'll get your money when you level. This comes from a post Rupert made, but since I can't be bothered finding it, might as well just tell you ^_^