This substance freely floats in space and is very rare in origin. It exists, or sends out, subspace waves of some kind, almost totally undetectable. It does not harm any living being, or damage planets or ships, and is so light a substance that it travels through any object. It may float through a person and this person will never know, nor experience any harm. However, when it comes in contact with computer systems that have an artificial intelligence of any degree at all, it interferes with the computer's normal function patterns.
When the Black Wave comes in contact with such a computer system, there is no way to predict what the computer system will do next. Its algorythms will be so messed up that anything can happen, that the computer controls. If the ship's computer system controls the weaponry, it can suddenly target weapons and fire at will, without anyone having given the order. When this happens, it is very difficult to prove that this was indeed caused by the Black Wave, since when once it has passed through, it leaves practically no residue, leaving little possibilities to scan for it afterwards.
The only way to prevent from the computer system to perform disastrous acts, is to shut down the computer's artificial intelligence. That way there is little to no danger for interference with the subspace waves.