Answer
Due to the position of center of gravity or CG
Work Step by Step
In a tapered water glass, the center of gravity is located slightly on the upper side because of the large diameter above and narrow base below. When such a glass is filled, the center of gravity starts shifting more upwards and whenever the CG becomes too high, the object becomes unstable and thus in this case the tapered water glass filled with water topples over.
Whereas in a glass with uniform straight sides, it doesn't matter whether the glass is full or empty since the CG here is approximately at the middle portion of the glass and even when the water starts filling, the position of CG does not change considerably .
