To derive again from ground — recomputing from source rather than trusting the cached; the dynamics' own law.
Re- (Latin, again) attached to derive, from Latin derivare (to lead off, to draw from a stream): de- (from) + rivus (a stream); to rederive is to draw the water again from the source rather than trust what was already carried away in a bucket.
To derive again from the ground rather than trust what was cached. The root's stream is the right image: a rederivation goes back to the source rather than relying on what was already drawn off.