I checked your course and it looks like the last activity was performed on your course was on Aug 20th 2015, and as you already know that if a course is inactive for more than 15 days then the exercise system tenant of that course would be reset anytime after 15 days. So this is the main reason behind your exercise system got reset.
Please note that performing any of the below steps will keep your course active:
1. Before course completion: Completing a lesson within 15 days will be considered an activity
2. After course completion: Reset quiz and complete a quiz within 15 days will be considered activity
You can also track of your course's last progress by looking at the Dashboard. If this is within 15 days, then your exercise tenant is safe and will not be marked for deletion.