Why should the secondary of a power transformer not to be left on an open circuit?

In general there is no reason why it should not be, though there may be special situations. Perhaps there is confusion with CURRENT Transformers which must not be left open circuit because of the high voltages which they will produce. it may trip the CB. although there is no faults but a high current called Inrush current is flow when the the power transfomer energized while its secondry circuit is open