Most long run:
First mistake I saw was how you contracted your left pec AND left hip. This is not good, it is better to extend the left pec, contracting it may help you for that step, but it will ruin your run in the later steps. It is also unrealistic. Remember, when right leg is forward, right arm is back. When left leg is foward, left arm is back.
Next mistake is relaxing wrists and elbows, since you are a beginning, I recomend you keep these two pair of joints on hold throughout the run. Once you get to a higher level where you really understand how a run works, then you can try relaxing your elbows. Even I have not yet understood fully how it works, you got to consider factors such are being assymetrical, torque on chest factor, speed and momentum of rotating pecs etc....it's really complicated. so just to be safe, keep your elbows below 90 degrees.
Your first 3 steps were good. Here is the reason why you fell:
You let the angle of your back go too low. This puts too much weight to the front and causes you to fall forward, reason for this is understepping, you did not put your leg forst enough. At around 790 frames, you held your left knee and a pretty acute angle. Cause you to understep when your extended your hip. You can easily fix this by extending your knee for 1 frame then relax. This allows your foot to step more forward and prevent you from falling forward.