Shackle angle?

Here's the thing. When a leaf is compressed to the maximum, as like when you hit a bump or overload the trunk, they become FLATTER and LONGER. This will force the shackles as far to the rear (at the bottom of the shackle) as they will get.

BUT when you jack up the car and let the rear axle hang, OR you are driving, go over a huge bump and unload the rear axle, IE so bad that the axle might even become airborne (Come ON we've ALL done it!!!)

At that point the springs have maximum arch and are the SHORTEST so the shackles are FORWARD at the bottom

Now in the case of poor design, spring mods, different springs, relocated shackle mounts, etc, etc, You can reach a point where the shackles go "over center" and snap back in such a way, ---front or rear--- that the bottom of the shackle is reversed and up against the frame.

It's important to realize that this can happen if the springs are excessively short OR long in relation to the mounts

And, according to the manual Stroker mentioned above:

With the car sitting at static (level) the rear shackle should have the bottom spring eye slightly to the rear of the top pivot, or acceptable if vertical, NEVER with the lower eye in front of the top pivot.