Question: who tipped filch off about dungbombs?
Answer: Norris spots Harry and runs to find Filch. Filch comes storming into the Owlery accusing Harry of ordering Dungbombs against school rules. This is completely not true, and later, Hermione puts together that someone (a.k.a. Professor Umbridge) tipped Filch off in the hopes that he would search Harry's mail.