Why was the bull a symbol of Poseidon?

Answer 1

The bull from the sea is the earthquake, caused by Poseidon in his role as earth shaker.

The bull was an important creature, respected for its power, ferocity and virility and was often the animal sacrificed in Homer.

Answer 2

No, not really, however there is a myth regarding Poseidon and the Bull of Crete.

It goes like so;

According to Acusilaus, this bull was the same as the one which had carried Europa across the sea; according to others, he had been sent out of the sea by Poseidon, that Minos might sacrifice him to the god of the sea. But Minos was so charmed with the beauty of the animal, that he kept it, and sacrificed another in its stead. Poseidon punished Minos, by making the fine bull mad, and causing it to make great havoc in the island. Heracles was ordered by Eurystheus to catch the bull, and Minos, of course, willingly allowed him to do so. Heracles accomplished the task, and brought the bull home on his shoulders, but he then set the animal free again. The bull now roamed about through Greece, and at last came to Marathon, where we meet it again in the stories of Theseus.

This bull is also said to be the father of the Minotaur by Queen Pasiphae of Krete after she had coupled with a bull.