Exactly what do you mean by "cocked the wheel"?
I'd check the surfaces of what'd be the locknuts on a regular wheel. See if there's any "bite" in those.
That configuration, you have brake forces pushing the axle back, then you have the drive forces pulling it forward. If there's any slop in the axle assembly, and if the t/a isn't closed firmly enough, the wheel will work itself loose as soon as it starts sliding.