Working for easyJet and not Jet 2, whenever I am on an aircraft where the APU is US and we are having a GPU connected I always say something along the lines of "On arrival on stand, there will be a slight delay before the engines are shut down and the fasten seat belt signs are switched off as the aircraft will be being connected to an external electrical source. Please remain seated with seat belts fastened until the signs above you have been switched off". I do this because otherwise pax will be out of their seats and queueing at the forward door to get off before the engines have been shut down.
Pax do like to unfasten their seat belts and move towards the doors when the aircraft isn't quite on stand, and as the engines are running I don't particularly want 156 people standing in the aisles or in front of me before the aircraft is safely on stand properly.
Also, the aircraft will not be on chox until the engines are shut down, so there is a potential for movement and sudden braking before the engines are shut down properly - I don't particularly fancy having pax falling over and hurting themselves when they should still be seated (I hate doing the paperwork for injured pax!
).