You're going to have to work on keeping the back leg soft so you can absorb the weight with the hips straight down. You're also slightly over-reaching the back foot and not stepping out enough with the front so that brings your hips back behind the bar rather than straight down. That is also causing you lower back to hyperextend which is something you don't want to happen with maximum effort weights overhead.
A great exercise to help correct this is Split Push Presses
. You can see in the video that you want to let the back knee bend (while keeping it rigid) to lower the hips and achieve a strong lockout with a straight torso.