Why does my credit balance go down and then come back up?

That is the reservation system working as intended. When a generation starts, we reserve an estimate of its cost so a run can never spend credits you do not have. Your visible balance drops by the full reserved amount right away.

The estimate is deliberately on the high side. As the video is built, the actual credits used are taken from the reservation, and when the run finishes, whatever was not needed is released back to your balance immediately. That is why you may see your balance dip while videos are generating and then bounce back up once they complete.

You are only ever charged for what your video actually used, and only for successful generations. If a run fails, the whole reservation is released.