We have seen whenever Bhūmī-devī is in trouble, she assumes the form of a cow. Why is that so?

In the Goloka Vṛndāvana, there are Surabhi cows, also known as Kāma-dhenus (desire cows) which give anything one wants, just like kalpa-vṛkṣas, desire trees.

A material version of such cows are also there in the heavenly planets. Indra has the chief of them. He came with the Surabhi cow to Kṛṣṇa when he came down to apologize to Him after the Govardhana-līlā.

Since the earth (the bhū-maṇḍala plane) provides everything for the perfection of life, not only for living here, but also for performing sacrifices and thus to ascend to higher realms and even the spiritual world, she is like the Surabhi cow which fulfills all desires. So the deity of the earth has that form as well, as the cow.

Also, among the 7 mothers mentioned by Cāṇakya Paṇḍita, the cow and the earth are counted. So both are nondifferent, being the mother.