It is very common for ratings to increase or decrease without an obvious reason to the tester. This is because your work alone is not the only deciding factor to determine your rating. Your Tester Rating is calculated using an algorithm measuring your activity and quality of work within cycles during a set period of time compared to all rated testers within the community. This means that the activity and quality of work of other testers will have a major effect on your rating.
There is no need to be concerned if you see your rating decrease. Tester Ratings are designed to fluctuate. The best way to increase your rating is to continue testing and finding as many high value bugs as you can while producing quality bug reports.