NFHS incidental contact?

[marvin]

NFHS 2-36

SECTION 36 OBSTRUCTION (DEFENSE)
Obstruction is the act of the defensive team member that hinders or impedes a batter's attempt to make contact with a pitched ball or that impedes the progress of a runner or batter-runner who is legally running bases, unless the fielder is in possession of the ball or is making the initial play on a batted ball. The act may be intentional or unintentional, physical or verbal.

If the fielder, in your judgment, is making the initial play on a batted ball you would have interference. If the fielder was making the initial play it does not matter what the runner could have anticipated.

One of the exceptions that allows an obstructed runner to still be out is:

6. When the batter-runner is obstructed between two bases and she flies out.

So if you had judged obstruction on the play you described the batter is still out because of the caught fly ball.

It all boils down to whether, in the judgment of the umpire, the player that the batter/runner collided with was making an initial play on the batted ball. There does not have to be intent for interference to occur.