Why is it that you think terrors occur during sleep paralysis?

Bisectional. Your mind/body is only partially awake and that is why it reads terrors. Your conscious part is trying to call for action, for example, to move yourself out of the bed, but unfortunately, your leg muscles controlled by the unconscious part of your brain would not follow the call. You undoubtedly felt your legs are bound/confined etc.
Terrors/Paralysis. Because sleep paralysis occurs during the rem stage of sleep, when dreaming occurs, this is why your bad dreams occur during rem. You dream in rem. You are 'paralyzed' in rem. Nightmares occur in rem.