Cheating charge: Why was Matt Prior appealing ?

Matt Prior is accused of cheating. The England wicketkeeper caught a delivery from Michael Yardy after Johnson went for a big hit. Johnson's back foot was firmly planted in the crease, but Prior appealed for a clean bowling after actually knocking the bails off himself.

The balls grazes the bail on the way into Prior's gloves and this led the keeper to think that it had bowled Johnson.

6th ODI, SCG, Feb 2011