Your squat posture translates to the erg.

This posture is what leads to back pain, rib fractures, IT-band syndrome, and the list keeps going. The column-core process over time corrects this "survival" spine posture. This is such a common rowing posture and leads to all the training related injuries I see (more than 1,000 rowers). 

If you continue to use the same movement process you have been using your whole life you will continue to have recurring injuries.
The column-core training process changes how you move your body.  It changes the way you move. It trains you to keep your spine stable and shift all the work to the arms and legs.

Take a picure of your catch position and your squat posture and compare them.