I always thought it was called Scrum because when doing Agile development, typically everyone has a 5 min stand up meeting some time during the day. Normally when people are standing around, then tend to arrange in a circle, and since you have a group of people in a circle moving a project (ball) towards it's goal (down the field), it resembles a scrum in rugby.

Of course, this definition was imbued on me many many moons ago before the consultants hijacked the language.