Crew need to man the doors if refuelling so it's just easier to get them all off.
Pretty sure it's mandated under DfT rules that they need to reclear security before they can depart on a double drop run. It's why they really went out of fashion.Watched a 1/2 load on an Air Transat A330 sleepily troop off at Glasgow, go up a flight of stairs through gate security and then join the queue to reboard the same aircraft bound for MAN.