It's one of those things that are obvious in retrospect, but computers have had accelerometers used for sudden motion sensing for decades now, and most manufacturers employ this tech in some form or the other in their upper-end modules.
Yet it took one bright individual to figure out that there's no point in using the accelerometer just for SMS, when in fact, its API can be exposed to the world for whatever other accelerometer-related needs. I don't know why other manufacturers haven't followed suite, but given the state of the industry I would not be surprised if patents are, in one way or the other, to blame.
I assume it's really the other way around—a "sudden-drop detector" is nothing more than an accelerometer.