Also possible bolt/stud vibrated loose from adjuster block and backed out. Two possible causes of what happened.
1.
adjuster-block stud - not really bolt because it doesn't have head and it doesn't spin. How did you install stud into adjuster block? Did you fully thread it in all the way and bottom out threads? A!so use red loctite to install stud fully into adjuster block so it doesn't back out.
On some bikes, adjuster bolt screws in & out of block to position axle. But not on this one; stud should be firmly affixed to adjuster block. Then larger nut moves block to position axle. Smaller nut locks big nut in place. Axle nut clamps whole thing togther.
2.
axle-nut - yeah, axle may not be tight enough. What torque did you tighten it to? Really need a HUGE torque wrench to get it tight enough. With 18" breaker-bar, I have to add a 2-ft extension pipe. Look up torque specs in manual and get it that tight!