It was in the 1st qtr, and he made contact with Fisher's face with the pointy part of his elbow.It was only a glancing blow, with not a lot of contact. Fisher had moved in front of him to shepherd, and Petts lifted his right arm. He then sort of apologised to Fisher.
He lost his head for a moment. He possibly anticipated forceful contact from Fisher, and lifted his arm to ward him off. It's also possible he tried to take it away at the last moment, but couldn't manage to get his arm clear. No real damage done to Fisher.
I think he'll be suspended for a match - maybe more with those suspended points.