As a general point in icing condition many jet types have a requirement to run up the engines to a specified RPM or EPR value at stated intervals when on the ground, e.g. every 20 minutes/every hour...and there's also usually a pre-takeoff run up required, sounds like that is what was being talked about....
As has been said it is done to ensure the engines are free of ice, see my last para.
Out of interest why are you assuming "TOGA" wasn't used? TOGA mode is not just an engine setting, it's actually an automation mode and is tied in with the nav kit, engines, and other things....not using it can cause some interesting complications to your working day. Generally once your pre take off de-icing timed run-up is done you do hit the TOGA switches to let the systems know you are starting your takeoff, as well getting the engines set to takeoff power. Another point is the power setting for de-icing can be quite high and I'm assuming you were in the cabin - I'm not sure you would always be able to ascertain the change from a de-ice run up RPM/EPR to the take off setting in all circumstances, so all in all I suspect TOGA mode was used.
There's probably more than enough info for you here, including comments on ice shedding:
Engine Core and Fan De/Anti-icing - SKYbrary Aviation Safety