Numerous apps tend to use BT and it can not only be a big battery drain, but a security risk. The new iOS version shuts down most of those, and requires to you okay them as they come up. I think (haven't played with it enough yet), that if you do allow it once, it 'sticks', so is only a one-time thing.
If your phone is continuously trying to ping the watch, or vice-versa, and is not allowed or paired successfully, it tends to use higher power to 'search' for it rather than the typically shorter range, and thus power when you phone is in your pocket and the watch is on your wrist, shortening the battery life.