Wheels coming off !! (TM2)

[Benwoods]

Hi all ,
I experienced my TM2 backing rear wheel nuts loose and even in 1 heat a wheel coming off . Using serrated nuts and tightened them just before the heat.
Running on high grip indoor astro.
Could this be slipper too tight ?or diff issues ?.
Using new team c wheels and standard axels / 12mm hexes.
I'm a bit baffled and don't want it to keep happening .
Any ideas ?