What are the chances of two of our COL'ers having this happen to their fur babies...... 
Hope Jemima makes a full and speedy recovery 
From what Sheryl's surgeon has told her and from what I have since read this is my understanding of why cockers are at risk.
Disc degeneration / disease has two forms, one that affects Non Chondrodystrophoid dogs (larger breeds) and one that affects Chondrodystrophoid (smaller / dwarf breeds).
The Chondrodystrophoid group sub divides further into Disproportionate Chondrodystrophoid breeds ie dachshunds, basset hounds etc which have long backs and short legs and Proportionate Chondrodystrophoid breeds which outwardly look normal. Cockers are in the latter group.
In dogs from the chondrodystrophoid groups their intervertebral discs start to harden at an accelerated rate fairly early on in life and since they are still so active it makes them more prone to slipping and rupturing. They generally present with symptoms around the ages of 3-7 yrs.