Well I heard that Imam Ahmad was a muhaddith, and he didn't like people writing down anything besdies ahadith. However, when the mu'tazili fitnah occured, and our Imam was beaten, his popularity rised and students flocked to him. Some of his students disobeyed his command not to consign to writing his usul, after he passed away, and hence his madh-hab survived. Wa Llahu a'lam.