He initially touched the ball behind the line of scrimmage, even though he didn't secure the ball until beyond. And yes there was a lineman down field, and yes that's why the flag was thrown.

And the flag was picked up when it was determined the passed ball was first touched behind the line of scrimmage.

And the same rules apply to a QB, WRT a passed ball being caught behind the line of scrimmage irregardless of where the linemen are on the field.