Digimesh network and unexpected hardware reset

I am creating a Digimesh network using Xbee pro (S1 and S2C).
Performing FN commands some of the Xbees get stucked and return a Modem Status Frame 0x8A, with the status “Hardware reset” 0x00. There is no way to get out of it. I tried the FR command but nothing happens. The solution I found is simply to turn of and on again the Xbee module.
Why this could happen?