That could be a result of the signal line becoming physically connected just before the ground line when plugging in the 3.5 mm plug at the sound card due to the design of the plug. Then any voltage differential between the "ground" of your external amp and "ground" the computer (known as ground-loop voltage) would briefly try to equalise itself across the signal line, destroying sound hardware in the process. A separate grounding wire connecting the two should solve the problem, (but will not resurrect the sound hardware).
http://www.epanorama.net/documents/groundloop/