I believe it shust off as part of the critical suspend. If customers are seeing it shut down prior to that it sounds possible that they may have went into a critical suspend, then cradled the device to give it just enough power to come out and then to go back in. I agree this is a guess but this does may be a likely scenario. It also could be related to the SPR someone mentioned earlier. Either way, I would suggest gathering some monitoring logs to see if we can pick up any cases where the radio is shutting down prior to the suspend. Hope this helps.

but I have a lot of queries from partners and customers that the cellular radio shuts of BEFORE the device shuts off ...

There have been a number of SPR's for a problem where the radio randomly shuts itself off. I dont know what the final resolution was, but my customers are still running a symscript to monitor the state of the WAN radio and re-enable it if it turns off. ON the MC55 we saw some issues when the battey died completely so we modified the script to no longer automatically re-enable the radio at somewhere around 20% of power...attached is the cab file containing  the script to monitor he radio and turn ot back on if t goes off

I worked with ECSG on a run time / battery life tool and the battery should put the device (and the radio) in standby mode when the battery reached 11% of total capacity. This applies to the current battery technology in the MC7x, MC9x, MC5x, Mc65, etc. This state is called device critical suspend state..