Other mistake: After the level crossing, the back of Stromwell's car is missing entirely the fender or any sort of lights. It's a close-up, even. (00:05:45)
Other mistake: Batman leaps off a tall building carrying Stromwell and lands on one foot without breaking the fall. (00:12:25)
Other mistake: The Stromwells are hugging in front of the office building, but that was the starting point 5 minutes before, there has been a long chase since. (00:21:15)
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.