why is the earth bulged at the equator

The bulging at equator and flattening of poles is due to the rotation of the Earth. The rotation of the Earth creates a centrifugal force which acts radially outwards and is a pseudo effect counter balancing the inward acting centripetal force. This pseudo effect basically pushes the Earth outwards creating the bulge at the equator and flattening of the poles. Its similar to squashing a rubber ball.