I want to contrive a scenario in which 23rd century humanity faces a dire crisis due to an extraterrestrial von Neumann ecology that rapidly consumes Nitrogen as a part of its life cycle. So rapidly in fact that it threatens to deplete Titan's Nitrogen atmosphere in under a few hundred years, potentially starving the millions of orbiting habitats around Earth and Mars which rely on nitrogen imported from the outer Solar System to grow their food. Scientists fear the tiny machines may even adapt to living inside a human digestive system if allowed to spread to human-occupied territory, sucking the Nitrogen out of the food processed by our digestive systems before our bodies have a chance to use it. Effectively, we would all die of malnutrition, starvation, or both if these things are not stopped.
Now the biggest question is, who would build this kind of thing and why? Clearly the answer to the first half is an alien species whose biology is not reliant on Nitrogen in any significant way and who cannot or simply failed to conceive of a species like humanity that is extremely reliant on it. But the second half is the most important part. Why? What advantages does creating an ecology of Nitrogen-eating machines provide you?
My best guess is that nitrogen is extremely abundant in the interstellar medium and a species that is not reliant on it might see no problems in taking advantage of this to help their probes spread throughout interstellar space, because they simply do not see Nitrogen as having any value relative to its abundance. However I am not sure if this is a good enough answer to stand up to scientific rigor. Are there any other possible justifications for this?