What I figured it was is that the system that had this happen had been previously treated with copper. I have been soaking it out of the water as the substrate and rock leeched it with Poly Filter and Carbon, and I figured that the copper levels had finally dropped to the point where inverts were able to survive again.
At the same time, I did a large water change about a week ago, larger than normal, which may have tripped it off... dunno