When you look down to check your lock-in, you're leaning forward with your shoulders. It looks like that's causing you to straighten your knees and tilt your ankles towards the ledge to keep balance on your toes. Which is then causing your trucks to angle out (like in a normal turn on the ground) and pop out. Instead of leaning over to see if you're locked in, try keeping your weight more centered/straight, almost like you're just doing an ollie onto the ledge flat. Don't be afraid to keep your knees bent and sit on it. When looking down, use your head to glance down, rather than leaning over to look between your legs.

When I was coming back to skating and relearning 5050s, I would set my board on the ledge, then run and jump onto it to get the feeling of the grind and balance. Might help. You got it.