Why are Jockeys sometimes called Hoops?

Answer 1

Contrary to popular belief, it has nothing to do with their race silks.

I am led to believe that it is a term originating from British hunting, and finding its way into racing vernacular (especially in Australian slang).

The jockey is referred to as a "Hoop" as that is what they look like if they tumble from their mount and cart-wheel along the turf.