I am going to speculate, since it was asked why.

When the rear end gears break, the trans goes from loaded to unloaded. Then as the rear parts catch a tooth or something, it gets loaded again. This ON-OFF-ON action is what can "disrupt" the rollers/springs in the sprag. This is why manual reverse valve bodies with no rear band apply in 1st need special care when driving in 1st.

Once those rollers start flying around in the trans, something can easily get hung up and the torque of the motor shocking the aluminum splines in the front planetary can strip them right out (output shaft jammed, motor going 6000 rpm suddenly all most stopping except for the torque converter).

Then again, maybe the splines stripped first, causeing weird rotations in the rotating parts that allowed the sprag to come apart.