It sounds like you was in the wrong gear for the gradient?
Down hill the crude Mazda system does not use the brakes in CC, so speed can increase, even the auto seems to not want to use engine braking at high revs, I often use manual to override it, both solo and towing.
I should also mention that the manual states something like "don't use CC in hilly terrain" or I could have confused that with the Nissan system (cruder than the Mazda CC), but there are a few restrictions to using CC,heavy rain for example.