The ON tells us, as the carriages depart Randalls, "Isabella stept in [to the Woodhouse carriage] after her father; John Knightley, forgetting that he did not belong to their party, stept in after his wife very naturally; so that Emma found, on being escorted and followed into the second carriage by Mr. Elton....and that they were to have a tête-á-tête drive." If she says so I must believe it - otherwise I would be disposed to wonder whether John Knightley did not do it on purpose. He is in a mischievous mood that night, having already wound up Mr Woodhouse by exaggerating the extent of the snow. He might have thought, after Emma's brushing off his warning about Mr Elton, that it would serve her right to give Mr Elton a golden opportunity to declare himself and prove he had been right. No serious harm could come to Emma on a short journey with the coachman within call, so he might not feel too guilty about abandoning her to Mr Elton's tender mercies.