There are so many variables here that it would be impossible to itemize each one and do justice to that effort. How you pitch physically, your physical condition, and a host of other things all influence your overall choreography.
Here are two very common aspects that can influence your finish posture(s).
THROWING ACROSS YOURSELF
A common influence is landing with your stride leg/foot. As you can see from the picture below, this can cause the lower part of your body going in one direction, while the upper part of your body is going yet another direction - hence, a pitcher can throw across himself/herself. The after results, tends to twist the weight of the upper body in a circle, thus for a right handed pitcher, that weight tends to move towards first base and the rest of the body follows.
HOLES ON THE MOUND
For a right handed pitcher, a hole that is dug in front of the rubber can be found with that hole slanting downward toward the third baseline. Hence, during the leg lift, the body's weight is in motion towards the third baseline all because the pivot foot is leaning the entire body off to third base.
In addition, a hole that made in the front part of the mound can cause the landing of the stride foot to be unsteady, thus causing the body to balance itself off.... again, shift weight to the first baseline.