Those random shoes...

WTF is with those random single shoes you see tossed on the side of the road?  Where did they come from?  How did they get there?  I can't count how many of these shoes I've seen over the years but it seems to be increasing or perhaps I am looking for them now - like when you buy a certain type of car and suddenly notice them everywhere.  I feel like if I figure this out, the secret of life will be revealed to me.