Not his job.
Let Stones fails his, dont fail yours then excuse yourself. Walker becomes the right back which is his job and gets out, Stones and Guehi are responsible for that area. He doesnt do his job plain and simple. And it happened more than once, not just on the first goal.
The point of wingbacks is to defend higher up the pitch and support the attack. Not to sit back. Thats a back 5.
Stones is in position to defend in that clip. Walker is not in position to take on his RB responsibility which is part of his duties as a RCB. Thats the end of it