Random...that's about all I can really say.
I guess it's nice someone picked it up and put it there, maybe in hopes that the owner of the hubcap would drive by and go "Oh, there it is!"
Actually you'd be surprised at the amount of crap I pick up and toss to the side when walking just so cars don't hit it. So think of that next time your waiting for a pedestrian to cross the street and get impatient.
Hell, the other day I actually had to walk in the middle of the street and hold up my hands so cars would stop...a big chunk of metal fell off a truck and would have easily punctured a tire, dude was wailing on his horn and looked like he was yelling at me, I reached down picked it up, showed it to him, threw it off to the side and flipped him off...haha. Yeah, I did it to help you, ass.