That's odd. I recall that they said that Joseph died because he didn't have the inherent mental blocks that Magneto had developed. So unrestricted use of the powers burned out his (Joseph's) body and he died.
Magneto's machine that he used as a generator to create Electromagnetic hurricane was damaged by Astra so when Joseph interfered Magneto was in the middle of sustaining those energies from being released into the Earth's magnetosphere, so if he would have released them he would "become living part of the EMStorm", now when X-men interrupted they removed him from the field of restraining these energies, yet it already affected him (his X-gene was damaged in the end), so Joseph was left to sustain those powers and the same explanation was applied on him as well, he could have embraced the energies to become one with the nature or either fight it and be damaged by them to save the magnetosphere, which he chose as a "noble hero and stuff". It was said these energies are too much for him to control, and that his organic matter started to decay. So I guess it was lack of knowing what exactly he's doing.