Why was a diesel bus used instead of a trolleybus?

It was due to the bad weather condition on this day and low temperatures. The overhead wires were frosted in the morning so I guess no classic trolleybuses were able to climb the hilly streets and instead of them all the buses and trolleybuses with alternative drive had to be used. And they stayed there for the whole day :) Also it was the last day of massive use of these Citybuses, because new Urbanways are now in use so they are not needed that much now.