Twelve volts would hurt in the mouth - maybe the wire was stuck in the cats mouth and it died of a heart attack? My second guess would be the transformer might have a high leakage current due to a defect or damage. Short of using a leakage current tester, you could test for this by measuring the current from the low voltage circuit to ground through 2000 ohms. There should be less than one milliamp (safety standards might allow more, but I've never seen anything over 1 mA in a good transformer). Just a guess.
