ah, the old "bridge crew flying from butt shot" question. Well, you were right that the ship's relative gravity field is unaffected by sudden jolts, but you have forgotten about inertia.
The crew and pretty much everything else aboard the ship that isn't bolted down still follow Newton's laws of inertia. Say the ship was at rest. All the items (and crew) would be at rest as well. But when the ship is moved suddenly (say a photon torpedo), the ship moves, but the crew are still technically "at rest" for a moment until they accelerate to the ship's velocity.
But this raises the question, "Why don't the crew get turned into chunky salsa when the ship (pretty much) instantly accelerates to hyperlight speeds?" This is explained using the Inertial Dampening Field. The IDF projects a 'counter-acceleration' ahead of the ship, which allows the vehicle to accelerate at a tremendous rate, whilst the crew are unaffected. However, this only applies to anticipated acceleration. A sudden, unexpected movement can send the crew flying because the computer must calculate and compensate with the IDF, a process that takes a few seconds. This explains the characteristic 'lag' that allows for the crew to be sent flying when the ship is hit by something.
EDIT: ack! hand cramp!
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