What was the voltage they ran on? the way you describe the issue tells me that your miners' PSUs weren't so happy with the voltage they had, either too high or too low.
208V, which is on the low end. The current draw into the PSU would be higher than if I had access to a higher voltage (higher voltages, hence lower currents, are generally more desirable). Having all the units go down in what appears to be the same way, over a narrow period of time, suggests to me the PSUs may have been driven too hard. I haven't taken one of the PSUs apart yet, but I'll do that just to see if there is anything to see. Now that I don't have any more working M20's, I don't have a trusted PSU I could swap in to see if the miner works.