It got too hot in the summer and too cold in the winter. New mammals ate their food. Mammals ate the dino eggs. Giant meteor smashed into Earth. Cosmic rays bombarded Earth.
Massive volcanoes explode all around the Earth.
They all joined a cult and committed mass suicide. Meat eaters ate the plant eaters, then starved. Ocean currents switched and put cold water where it used to be warm. New insects evolved which spread nasty diseases. Dinos got stressed out and laid eggs with thin shells.