The pilots may have missed their slot time and had to negotiate for another one which was an hour later. With the need to de-ice coupled with a slot time the guys have to figure out when to start push back because you don't want to bust your holdover time and have go back for a 2nd squirt thereby missing your slot time and have to renegotiate for yet another one. So now considerations are taxi time, traffic, position of the de-icing equiptment and any other time related factors you may have. From all these considerations you choose what you think is the best time to push back from the gate so that you 1. dont bust your holdover time after being de-iced and 2. maintain your slot time. Places like Moscow will de-ice you after you push back and then you have to taxi to the runway for the takeoff (possibly in a long queue) whereas in Amsterdam they de-ice you near the holding point of the runway so you can immediately take off.