What happened to Dr. Jekyll at Regent Park?

Dr. Jekyll turned into Mr. Hyde without the use of his potion..... Hyde began to take over on his own.

It was a fine, clear, January day, wet under foot where the frost had melted, but cloudless overhead; and the Regent’s Park was full of winter chirrupings and sweet with spring odours. I sat in the sun on a bench; the animal within me licking the chops of memory; the spiritual side a little drowsed, promising subsequent penitence, but not yet moved to begin. After all, I reflected, I was like my neighbours; and then I smiled, comparing myself with other men, comparing my active good-will with the lazy cruelty of their neglect. And at the very moment of that vainglorious thought, a qualm came over me, a horrid nausea and the most deadly shuddering. These passed away, and left me faint; and then as in its turn faintness subsided, I began to be aware of a change in the temper of my thoughts, a greater boldness, a contempt of danger, a solution of the bonds of obligation. I looked down; my clothes hung formlessly on my shrunken limbs; the hand that lay on my knee was corded and hairy. I was once more Edward Hyde.