Why is Muhammad’s turning from Jerusalem and facing Mecca called a symbolic break with the past?
I assume that you are asking about the change in the direction in which Muslims were supposed to pray. If so, this can be called a symbolic break with the past because it deemphasizes the importance of Judaism (the first monotheistic religion) and instead asserts the importance of Islam (the new monotheistic religion).
Muhammad understood himself as a prophet in the monotheistic tradition that had started with Judaism and continued with Christianity. He believed that his revelations were meant to reform the monotheistic faith, which had strayed from what God really intended. It would make sense, then, that Muhammad would have originally emphasized the importance of Jerusalem. Jerusalem was the most holy city for Judaism and Christianity and praying towards Jerusalem would underline the idea that Islam was part of the same tradition as those faiths.
Later, Muhammad announced the change to face Mecca. We can say that this represents a break with the past because he no longer wants to emphasize that Islam is connected to the other two monotheistic faiths. You could say that Judaism and Christianity were monotheism’s past while Islam was its future. By turning away from the old center of monotheism, Muhammad was symbolically breaking with the past. He was turning away from the teachings of the older religions and turning towards the teaching of the new faith, Islam.