Leaf Spring Suspsension - 1964 Valiant

If the shackle won't stay in the more vertical position then there's another problem...springs too short, springs with too high an arch, or something else. You kinda have to keep pressure on the shackle while you lower the weight of the car onto the axle because it will try and stay in the wrong position. If the spring is the correct length it will "pop" back into the right location if you push the shackle down as you have someone lower the weight of the car onto the springs.
About the only way the shackles can accidentally get into that position is if you hit a big bump at higher speeds causing the rear end to lift off the ground which can allow the shackles to pop into that higher position.
Treblig