why did hyde appear without the help of the potion?

While asleep one night, he involuntarily transformed into Hyde, without the help of the potion. This is never fully explained. We can surmise that his darker half was taking over. He does turn back into Jekyll but the lure was too strong. He took the potion again and the effects were stronger and, it seemed, harder to turn back into his former self.

Jekyll is exercising his evil side more, so Hyde is becoming stronger-strong enough to appear without the help of the potion