Shoefiti, the modern mystery of dangling shoes
There are plenty theories about why people dangle shoes, but none of them have ever been fully vindicated.
Most prominent in de US is the explanation that dangling shoes signify a place to buy drugs or the area beneath the shoes is controlled by some form of organized crime.
I observed many skatespots in the Netherlands, where you can find the same phenomenom. While observing these spots, skaters gave me different reasons for the shoefiti. Mostly it is because they want to tag the place with their worn-out shoes, but sometimes it is an ode to a deceased skater.
Since no-none is capable of pointing to a correct theory, than most likely all, or at least most, are correct, and of course only each individual shoethrower knows why his/her shoes hanging in trees or other high objects.