Round here it's usually bunches of flowers for somewhere someone has died, often by the roadside after a fatal accident. I can't remember the flowers when I was younger. Probably someone did it and others liked the idea and copied it. Now they are doing it everywhere.
Could be the same with the shoes.

Another idea - in some places there is a tradition of hanging bits of cloth to a particular tree, often in connection with a wish. Maybe they throw the shoes and make a wish. If the shoes stay on the wire the wish will be granted (maybe).