It's a brain stem response.
Embarrassed my daughter at her soccer game (she's the high school coach now). Watching a game from the stands, someone shanked one onto the walkway. I yelled, "KEEPER!", leapt up (ok, about 4 inches), and made a perfect high point save. Didn't realize what I'd done until my friend starting laughing.
Gotta think you're obligated to bring your mitt if you're sitting down the foul line. It's your civic duty.