Rear axle adjuster block missing

Have a 2011 570 and was out for a ride , was quite wet and muddy at times and as rough as bags also ,I felt like i was getting a flat through out the day, then heard a real bad crunching death noise and chain rattle , thought my gearbox had lost all its gears, pulled over, axle with nut and loose chain adjuster block still on was hanging right out , thought i had snapped the axle, went to the other side and saw no chain adjuster block , kicked the axle back in and saw the thread of the axle , lucky one of the blokes had a nut in his bag that fitted in the gap to hold the axle back and a further one that fitted on the axle , how lucky can you be .
ps i did throw the husaberg axle spanner on before starting and tension was good.
I am tipping one of the two has happened ...
-the alloy bit cracked and went
-the axle unscrewed itself and the block fell off- although there is no reason for this to happen
-Has this happened to anyone else ?
-has anyone got a fix ?
or just put a steel nut on it ?

Thanks for the replies ,
I did have the nut on the sprocket side because i like have the option to put my boot the spanner to tighten , I had a ktm prior and it never happened to me before ,
It did cross my mind that this may have happened , but i wouldnt have thought that it would have intent (friction) to turn the axle , all bearings are smooth and free, which made me think it had snapped.