RE: What is causing this?
Bows need to be completely tuned at least once each year, and that's if you have a good string and cable that doesn't creep. If you have a stock string, then tuning ususally needs to be much more frequent.
The easy way to check this, is the take measurements once your bow is tuned perfectly. Write down the exact brace height, ATA, distance of nocking point from an axel, draw weight, and the tiller. Every now and then, take the measurements and make sure they are precisely the same as before. If not exactly the same, then you need to re-tune. Periodically the string and cable need to be twisted or replaced to get back to these measurements.