It’s to do with the principle of moments and leverage.
If you imagine the entire tower being a lever, with the pivot point (centre of rotation) of the tower being the very bottom edge, the greater the perpendicular distance from the line of the action of the force on the tower to the pivot, the weaker the force of the pivot needed for the centre of mass to pass over the pivot. (Moment = Fd, F=force d=perpendicular distance from pivot to force).
Essentially in order to have the same moment, you can have a lot of force on a short lever, or a long lever with little force; which is how wrenches work.
Once the centre of mass is beyond the pivot, i.e. there is no supporting force on the centre of mass, gravity does the rest in the collapse.
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