Is that the linesman was calling Findley offsides... and I think it was a good call. When the ball pops back out towards Dempsey, it puts Findley in an offsides position. Since he popped up and tried to play the ball (even though Dempsey got there first), it can be interpreted that he interfered with the goalkeeper. Had he stayed on the back post where he was, he might not have gotten called for being offsides since he would have been out of the play.
At the very least, that's my guess as to what transpired.