why did professor McGonagall stop the boys as they were heading to their dormitories

They were out of the dormitory and past curfew. When they return to Hogwarts, Professor McGonagall discovers them in the entrance hall. She tells Ron and Harry that they will do their detentions that evening. Ron will be polishing silver with Mr. Filch, without the use of magic. Harry will be helping Prof. Lockhart answer his fan mail. Lockhart requested Harry particularly.