It IS explained. The AIC makes it totally clear that if you can't maintain a sensible distance from the aircraft in front, you break off and join the racetrack pattern at Malmesbury. Pilots must have been either fed up, impatient to get on the ground, or more likely hadn't read the AIC. Or even if they'd read and digested all 20 pages, they couldn't remember it all, so forgot the really essential bits. Yes, the AIC needs to be shorter...but we've been here before, haven't we.