You did the critical action absolutely right! its all going pear-shaped, GO AROUND! We glider pilots don't have that option, so we have a very bad habit when flying power to forget about that option, and don't practice it near enough.
On that day you state "wind was calm on approach." Which sounds very much to me like a thermal must have been lifting off at your critical moment. This can cause a surprising gust of wind from a completely random direction, on otherwise a flat calm day.