Climates changes occur for lots of reasons.
Near me it was a volcanic eruption 34 million years ago.

In the late Eocene to early Oligocene, approximately 34 million years ago, the area was a lake environment with redwood trees. The basement is the Proterozoic aged Pikes Peak Granite. There is an unconformity from the Pikes Peak Granite to the next unit, the Wall Mountain tuff.[9] The massive unconformity is due to erosion that occurred during the uplift of the modern Rocky Mountains, the Laramide Orogeny.[10] The Wall Mountain Tuff was deposited as a result of a large eruption from a distant caldera.