It hard when a horse wants to go running off NOT to grab the reins and use them to get the horse "together", but as you found out that has caused the horse to be behing the leg.
1 - Mouth open evading bit action - rider should: slightly give about 1/2 inch more rein, anchor elbows to waist and use legs to encourage horse to come forward and engage back. Best fixed at the trot since rider can slow their posting to the rythmn they want. At first horse will try to speed up - "you did use legs mom didn't you?" but if the rider persists in posting to the speed/rythmn they want the horse will slow their rythmn (eventually) in response but since rider is not allowing horse to get behind the legs (i.e. Squeezing forward as necessary) horse will end up taking larger steps rather than speeding up.
2 - Horse still behind the leg and not through. You can visually tell when a horse is correclt through because when they arch their neck and step underneath themselves it causes a "bulge" in the middle of their neck. No bulge? Horse is faking you out with a headset (not using their back so rider needs to put legs behind girth and drive horse into reins).
3 - This is the nicest picture. Horse is starting to use their back - stepping up and through. Notice you have a slightly longer rein length here. Dressage work is capable of being done on a longer rein - IF done correctly.
4, 5 - Horse again behind leg
6 - Notice you do NOT have a nice square halt (LH is out behind the horse). Work on getting that by using your legs - that's the first step in getting and keeping the horse in front of the leg.