Zhou Tian's action had indeed pushed the Human Emperor into a corner at that time.
Only then did the Human Emperor begin to realize something was amiss. Initially, he was solely focused on expanding his own territory and paid no attention to his internal affairs. As a result, when Zhou Tian schemed against him and significantly weakened his strength, the Human Emperor was still unaware that he had fallen victim to Zhou Tian's schemes.
So, when Zhou Tian launched an attack on his territory, it was only then that the Human Emperor realized, to his shock, that his territory had somehow become much weaker. Zhou Tian alone now had the ability to invade his territory, and under those circumstances, Zhou Tian's actions forced him to defend himself.