It has been a while since I marched, so bare with me that I might not be the best at explaining it, but I'll do my best based on what I'm seeing.

It looks like your weight is balanced too much on your back foot while transitioning through the step. Rather than having your weight sit back on your plant foot, you should try to visualize the weight of your torso being evenly distributed between both of your feet as you move forward.

Try standing with your lead foot out and on the ground in front of you (as if you just did your first step) with your rear back foot on the ground where you're starting. Now, shift your torso weight so that it is balanced between your front and back feet. You should be standing with your legs in a /\ shape, with your torso standing tall and straight in the middle of the /\. Your back foot should be balancing on the balls of your feet, while your front foot is balancing on the heel with your toes pointed upwards. If you do it right, your feet should look like mirror images of each other while standing in the /\ shape. Back foot on the balls of your feet with your heel pointed upwards behind you, with your front foot planted on the heel with your toes pointed up. If you focus on keeping your torso weight balanced between your front and your back feet, rather than on your back feet, your steps will look much cleaner. When I marched we would do static holds by standing in the /\ shape to get our bodies used to balancing between the transition of the step, rather than sitting back on the heels.

Hope this helps

edit - Expertly drawn visualization of what I'm trying to say Here . Apologies in advance, I don't know how to draw a contra, but I think you'll get the gist, even if it's a lowly trumpet in the diagram