三彌勒經疏 1.215

1.215

經曰「三千大千」以下，第三動地怖魔。謂眾生長眠生死故，不動者不悟長，故須動也。

From the sūtra passage stating, "The trichiliocosm," onward is the third, shaking the earth to terrify Māra. It means that because sentient beings slumber long in saṃsāra, without shaking they would not awaken from their long slumber; hence it was necessary to shake it.