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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

I see interesting, each country has its ways of expressing its gang way...
I could tell it had to do with expressing something but couldn't make up what it is, thanks for explaining.