If you’ve pitched your shoes in a similar manner, why?
We have similar things here in New Zealand, shoes in some, bras in others – or on fences.
As to who started them or why, I have no evidence.
I suspect that it is some aspect of a sense of fun, a sense of daring, a sense of the ridiculous, a sense of starting something that others might follow.
I do find it amusing to be driving in some far flung, out of the way place, and come around a corner to see a fence decorated with hundreds of bras – but that might just be my sense of humour, and a “having grown up in the 60s” sort of thing.
I have never myself contributed shoes, bras or any other sort of clothing item.
A few years ago we had something similar happen with rocks on one of our local rocky beaches.
Someone built a cairn. Someone else built another a bit larger. Within a few months there were thousands of little rocky spires 3 – 4 ft high. They’re all gone now. A big storm redistributed the lot of them.