We saw this fence when we visited Plain, WA, near Leavenworth. We couldn't find anyone to ask "Why?" - but we figured that someone had lost a boot, someone found it and stuck it on the fence - and then others started bringing old boots along and sticking them on the fence. Sounds good to me.
Just a big ol' cowboy boot
Looks like a hiking boot
Then somebody got silly and stuck an old tennis shoe up there.