They were propably on their out-lap after a red flag and were trying to get some distance between them and the car in front.Looks to me like some of the cars in that clip of GP2 were deliberately parking on the apex and going very slowly. Could just be perception at Monaco from external cameras though.
Plato twitted this video : GP2 Monaco Qualifying
Shocking driving
Just remember, those are the F1 drivers of tomorrow...