obstruction on a pickoff ?

[FTVMartin]

But the if the contact doesn't happen until the ball is nearly to the base then we have an immenent play. If a catcher is standing in the baseline while the ball is at the cutoff man of course it's obstruction but we don't have obstruction unless the runner is obstructed. By the time the pitcher makes his move and the starts coming back to first, the 1B is already in the act of making the play. Why is it now obstruction?