Jeremy - you are 100% correct. There are ways that you can minimize the risk (working out, warming-up, etc), but freak injuries do occur.
Jeff - A LOT of ACL injuries occur during the deceleration phase of a movement (basketball player landing after getting a rebound, soccer player planting their foot in order to cut and change directions, etc.) Those are movements that they have probably done a thousand times before...and it just happens.
This often occurs because there is weakness in the posterior chain and/or the quads are overdeveloped. (Usually the hamstrings, as is the case with a lot of females.)
Take a look at your local gym. When you actually do see people training their lower body (not very often), they are doing squats. Squats are a great exercise but they are a knee dominant exercise.
In order to balance out the squats you need to do a hip dominant exercise such as Romanian Deadlifts (aka straight leg deadlift), stability ball leg curls
(don't do them on a machine...), and stability ball back extensions
Here a couple of examples of hip dominant exercises...
Stability Ball Leg Curl
Stability Ball Back Extensions
If you need some more ideas for more hip dominant exercises, be sure to let me know.