Christmas With the Joker - S1-E38
Revealing mistake: Batman and Robin, thanks to the Joker's generous hint, are at the toy factory. In the wide shot just before the phrase "Look sharp, Robin", look at the shadow on the ground; Batman's cape deflates, but the shadow does not move and is still just as wide as it was before. (00:17:10)
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.