Both Darvish and catcher A.J. Pierzynski started heading towards the dugout. The ump saw the pitch differently and Darvish would walk the batter on the next pitch losing his bid for the perfect game (you can see the full video at MLB.com)...
Replays showed that the pitch was borderline but probably close enough that a pitcher working on a perfect game would normally get this call...
Fox Sports
To add insult to injury, the umpire appeared to have a short fuse and followed Pierzynski towards the mound after the walk and eventually ejected him...