Did the game crash, or did you close out of it normally?
If crashed, it likely opened up the crashlog in notepad or whatever your default text editor is, and steam thinks that's the game itself, cause the game launched it.
If not, steam wonkiness
I just close out normally through the menu (Just to test I tried just closing the game as well), of all my games this is the only game this happens to.
And it happens every time.
After googling it for a bit it seems to be a random problem with Steam and not this game though, people have had it happen to other games.