Why does it happen?

The wall of a dam is broad at its base.

Pressure of liquid increases with distance from the surface. So pressure at bottom part of dam will be much higher than at surface level. Higher pressure on narrow base area will results in collapse of dam. In order to avoid these, base area of walls of dam is made broader. So the force due to water is exerted on larger area and there will be only low pressure. Thus, the broader base area of dam causes less pressure and thereby keeps it safe for long time.