We did feed our cattle ground up animal protein at the time the one particular cow that got sick was younger.
BSE is a disease that causes lesions in the animal's brain. Clinical signs of BSE may include: Behavioral changes such as aggression or nervousness, abnormal posture, lack of coordination, inability or difficulty in rising or walking, decreased milk production, and weight loss with normal appetite. BSE is always fatal, normally within two weeks to six months after the signs first appear. However, the animal may be infected from two to eight years before showing clinical signs.
BSE is a disease hypothetically caused by a protein called a prion. Proteins are long chains of molecules called amino acids. The chains are all folded into a fancy shape due to the various attractions of the elements in the amino acids within the chain. For instance, one molecule's sulfur might be attracted to another molecule's oxygen atom 20 links down the chain, so the chain would fold in a loop at that point. The chain is hundreds of units long. Think of a tangled ball of string. The tangling is all planned by nature of course. The problem with the protein that causes BSE is that it can fold two different ways. If it folds the "good" way, then there's no problem. If it folds up the "bad" way, then it is the disease. Nobody knows what causes it to fold differently sometimes in some animals, and nobody knows how the bad folding causes the problem, though it seems bad ones can cause good ones to turn bad. This is all just a hypothesis that has been questioned due to it's rather slim proof so far. However, the theory was good enough to win Stanley B. Prusiner the 1997 Nobel prize in physiology and medicine.
More on BSE at Wikipedia.