Volcanoes are the only large source of additional carbon dioxide from "nature" and as the continents were moving "quickly" (after Pangaea split up), they were producing carbon dioxide more quickly than the weathering process could sink into sediments only up until India collided with Southeast Asia and started pushing up the Himalayan mountains. After that, the weathering process brought the level of carbon dioxide down to the 170-270 plateau that was the "naturally balanced" range that persisted right up until we humans started burning fossil fuels.