Potters boss Mark Hughes revealed Shawcross was booked in the dressing room at halftime by referee Lee Mason.
Mason followed Shawcross up the tunnel to show him a yellow card and Hughes said: "Ryan was booked in the dressing room. The referee made a point of knocking on the door and booking him. I've no idea why.
"He pushed him (Van Persie) I think, but I've no idea because I didn't see the incident. But he wanted to book Ryan. I've never known anyone get booked in the dressing room before."
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