Maurice is an intelligent cat.
The Rat King has the ability to control animals (he makes Maurice freeze and forces him to walk into the circle of huge rats) and see through their eyes. And when he gets angry, it causes mental pain to Maurice.
What he did was fill Maurice's mind with so much mental "static" that no mind was left there. Think of his mind as TV. Normally it shows a clear picture. But the Rat King used so much interference that the only thing left is static. And static for Maurice means "pure cat instincts" . Keep in mind Terry doesn't use the combination "intelligent cat" there. He uses the "clever cat" word combination.
A clever cat, but still... Just a cat. Nothing but a cat. All the way
to the forest and the cave, the fang and the claw... Just a cat. And
you can always trust a cat to be a cat.
Once the source of static (RatKing's anger) is removed, regular show continues for Maurice - which is thinking and intelligence.