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All slanted-parallelogram rear derailers are affected by bump forces, and unless the spring is stiff enough and the cable also straight and thus stiff enough, auto-shifting can occur in response to abrupt road defects.
But it's usually just momentary, and goes back to the same cog that the chain was on.
Unless that is: 1) the shift lever actuated or slipped or 2) the cabling and/or adjustment was sufficiently poor for the gear selection to be less robust and thus allow a "choice of gears" within any indexed position of the shift lever, wherein the derailer might fail to return accurately and immediately to it chosen gear position (cable friction being a possible factor here).
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