A woman tried to murder Jon. I don't think there's many other strips besides your weirdo alternative web comics that hinge gags on the attempted homicide of the main character. Then again, I don't read Marmaduke every day, so I could be wrong.
In this case, though we don't witness the scene, the situation probably wouldn't even be funny if we did. It would just be horrifying. Instead we are treated to this funny retelling, in which Jon looks depressed because his quest for love has failed, but also exhausted from running away from someone trying to run him down in a truck.
Garfield: not concerned for his owner's life, the cat just sees an opportunity to make fun of Jon's romantic track record.