A question that came from a beginner last night was “why is this hurting my back?”.
On the spot diagnosis proved that it was because she had hollowed out her back because she had not tilted her hips forward, keeping her sacrum low, torso straight. And because she was not extending back with all of her leg. Using only the ham and the knee instead of extending through the hip, the ham, the knee and the ankle. Essentially she was keeping her torso upright while asking her legs to go backwards and the lower back was taking the flack.
Should there be any other causes I should know about?

Additional cause.
Stand on the left extend back the right and then lean back the shoulders. If there is any weakness in the lower back you feel it immediately.
Also, if you are practicing ochos and the lead doesn’t wait for the wait change, there can be torquing.
and that’s the one lead that I tell followers to resist, although to-date it is because it takes the follower off their axis. I shall include the torquing in future.
I have sometimes given myself an ache in the lower back by ‘softening’ my knees a bit too much.
This is very tricky because it’s hard to explain to another person what she should do. I don’t know exactly what you mean by tilting the hips forward. Watch out for the effect that high heels have on that.
And it can happen with leaders who hold too tight, or those who hold fairly loosely but too low. Putting the hand at waist level tends to make the woman curve her back.
No I believe the leaders hand should never be below the diaphragm. Actually I am a proponent of quite a high right arm for the leader and would set the diaphragm as the 54th parallel.
Right you are LimerickTango.