"Newly formed solar systems can be violent places. Our own Moon is thought to have been created when a Mars-sized planet collided with the recently formed Earth and threw up a cloud of debris. A near collision during the early, violent stage of this planetary system could well have caused a gravitational slingshot, flinging WASP-17 into its backwards orbit."
But why is WASP-17 so big? The discovery team suggests that have been subjected to intense tides as it travelled in its strange and highly-elliptical orbit, causing it to become stretched and bloated.