fall back—
1. spring back; recoil:
- Mizuki watched Yukiko dig through her stuff, then she fell back when she saw her get the box.
2. (of troops) retreat:
- The army began falling back to prepared lines of defense.
3. lag behind; be outstripped:
- We thought the horse would win, but he fell back halfway through the race.
fall behind—
1. = fall back 3:
- After about twenty miles the horse slowly but surely falls behind.
2. fail to maintain a schedule of payments, etc.:
- If your payments of rent fall behind, you will be asked to leave.