Judge Ford thinks that Turtle set off the fireworks to divert suspicion from the real bomber, Angela. 

Turtle set off the fireworks in the elevator to protect her sister, Angela.  Theo called Turtle and told her that he saw Angela at the hospital.  Then he told her to let him have her bike.  The implication was obvious.  He was blackmailing her. 

Turtle hung up the phone. If Theo knew, others knew.  Angela had set off those fireworks wanting to get caught, but it was different now. Now she was confused, now she was just plain scared. They could force a confession out of her in no time, the guilt was right there staring out of those big blue eyes. (Ch. 21) 

Turtle's solution was dangerous, but she was desperate.  When the police come to investigate the “bombing” they find Turtle still in the hallway.  The police assure her mother that it was just a childish prank.  Turtle's mother is clueless about what both her daughters are up to.  The policeman found Turtle’s essay in the elevator, on the back of a sign that said, “THE BOMBER STRIKES AGAIN!!!” 

The police do not want the paperwork of taking Turtle in, so they take her to Judge Ford instead.  Judge Ford thinks she knows why Turtle would set off explosives in an elevator. 

She was protecting someone. She had set off the fireworks in the elevator to divert suspicion from the real bomber. But who was the real bomber? Nothing to do but drag it out of her, name by name, starting with the least likely. “Are you protecting Angela? (Ch. 21) 

Judge Ford is surprised at the empathic denial she gets from that suggestion.  It seems unlikely that someone like Angela would be the bomber, after all. She seems harmless.  Realizing she has underestimated Angela, as everyone else has, Judge Ford makes Turtle promise not to set off fireworks again.