Well,it might be because they're always 'piking'.......I.E. When they're going round collecting scrap (or whatever),their eyes are always looking everywhere,to see what else they can clock (for later !!).

Piking is,I think a Yorkshire term for looking........Hence they get called Pikeys !


** It's a variation on what Lee has said......i.e. most of 'em are theiving so and so's ! **