Of course the mast doesn't know what it's attached to; it's just a stupid mast. But the boat beneth the mast matters a lot whether or not it will break. Stepping it to a stationary object is not a good comparison. Also wind speed would be a constant in comparing the two boats. Wind doesn't increase when you put a mast up on a N20. Best case in point is downwind sailing, spin up. As you sail down wind the apparent wind decreases the load on the rig. The highest loads will be during acceleration, and when the boat suddenly slows stuffing into waves. Compound that with waves causing the top of the mast to whip. There are much higher stress loads than mounted on a stationary object. Up wind the apparent wind is increasing the wind velocity, add waves again and you'll have much higher stresses than stationary.

Mainly the this will cause over-stress on the mast: Downwind boatspeed is key, if you can maitain boatspeed close to the true wind speed the rig is not under stress even though the boat is moving fast. As waves slow the boat down, pressure on the rig increases. This will happen on a heavy boat to a greater extent than a light boat.