Ron had already been running out of the burrow and past the wards when Tom had started casting. The Lovegoods had a floo connection. With his mother in the house, he could not risk using theirs. It would betray the location of Harry's home. He could not do that, not even in a case of emergency. He nearly stumbled over Fred and George when they apparated in. Not caring what his brothers could want, he rounded them and sprinted on.