I think they took the idea from american gangs. There they throw shoes over power lines to indicate their gang "territory". But here, i think that the skaters think it looks cool. As for the decks that are up there, those are broken during stunts on that skate park, so they are up there as a memorial. You'll see broken decks around a lot of skate parks in the Netherlands
RE: A StreetArt trip along Dordrecht #20: Maasplaza SkatePark 2