If you were eating lunch in a school cafeteria when someone yelled, "Food fight!" and instead of hiding under the table you threw your hamburger across the room, you'd be a contributing factor to the ruckus, or something adding to it.
The word contributing can be used to describe things that help to bring about results, whether positive or negative. If you're a contributing member of your friend's fundraising efforts, it means you're giving her money. If you're a contributing voice in the audience at a noisy rock concert, it means you're screaming your head off.
