because in the game of rugby (where the term scrum comes from), the part where the game is won or lost is called the breakdown, which is a terrible name for a methodology you're trying to sell. So they call it the scrum, which although is useful and looks pretty cool, isn't really as important as other parts of the game.
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.
