Continuity mistake: When Catwoman first encounters Batman during the flashback, her whip is coiled around her shoulder. When the camera cuts back to Catwoman after the shot of Batman, the whip disappears altogether.
Other mistake: When Catwoman scratches Batman's face in the flashback, her fingers and claws stretch comically.
Continuity mistake: During the flashback, Catwoman's whip disappears and reappears on her belt in a couple of scenes.
Continuity mistake: Catwoman/Selina gains and loses her pupils throughout the episode.
Answer: Because he stole a chemical from his lab.
But Batman knew that with it, Clayface could permanently retain his shape and Clayface desperately wanted to be Matt Hagen again. In a sense, Batman stopped Clayface from becoming human.