Your muscles is built up by two different kind of fibers, white and red. The white ones are the largest and they are used when doing intense work, the red is used during more long lasting work.
When the red fibers work they will come to a certain point were they will need more energy because not enough is transported to them the regular way. So they do what is easiest, they break down the white ones so they get energy. That is why Cardio is worthless for body building and that is why HIIT training is good because the red fibers never reach that point where they scavange white muscle fibers.