Question

what causes slight flattering of the earth

Answer 1

Earth is rotating on it's axis once in 24 hours, and this translates to a speed on the surface of the Earth of ~1,600 km per hour. All parts of the Earth rotate at the same angular speed but as the latitude decreases, moving towards the equator, the radius of revolution  increases. Therefore the parts closer to the equator need larger centripetal forces to keep them revolving. To get those larger forces, the material of the Earth must "stretch out" more.

Answer 2

when the tectonic plates of the earth move then the earth use to move slightly also if there is a volcano eruptions is rally in high rate then it could be the second reason
hope it will help you