Liquid water tends to absorb microwave energy more strongly than frozen water (ice). So what happens is that part of your dinner thaws first, and then that part starts absorbing microwave energy far more quickly than the rest of your dinner. So if you just cook it on high for a couple of minutes, you’ll end up with parts of your dinner being smoking hot (possibly burnt) while other parts are still frozen solid.
By cooking on low first, there is time for the heat to move via conduction from the parts that thaw earliest to the parts that are still frozen. This way you defrost the entire dinner without incinerating local parts of it; once there’s no more ice, you crank up the power, and with the entire dinner having been thawed, it absorbs the microwave radiation much more evenly.
see thermal runaway.