Motor mount snapped on octocopter - could it be vibration?

Hey everyone, I have been flying an octocopter running 18" props. During an autotuning session today, one of my tarot motor mounts snapped clean off. Note, the mount was still connected to the arm, but the plate that connects the motor to the motor mount had broken. Could this be caused by excessive vibrations? Is there another potential cause? Any help would be greatly appreciated. Attached is the log file. Thanks all.