A. Two Shabbat rules are in play here. (1) Breaking twigs is not your desired intent as you walk across the grass, the goal is to get from Point A to Point B. (2) It is not absolutely certain that twigs will be broken. There is that (even remote) possibility that you won’t break any twigs on your walk across. For this to be permissible, both rules have to be in play, one rule is not enough to consider it permissible.