Since he is the Past, the Future and the Present and in fact, everything in the universe and since he is the origin of the past, the future and the present, he is for that reason called Bhava.
भूतं भव्यं भविष्यं च सर्वं जगदशेषतः। भव एव ततो यस्माद् भूतभव्यभवोद्भवः॥