Do ratings fall when users become inactive?

Personally from my experience, no, your rating shouldn't fall when you become inactive, I had a long period of inactiveness (usually in The Summer months) and I am still a blue star, always have been and I have been away longer than 3 weeks. It may be to do with your old discussions getting a negative response. As discussions can be responded to months or even years (yes I've had a year old discussion found and responded too!) later. But as you said no big deal. There are quite a few oddities that go on, I guess like me you are curious more than put off by it.

Ratings don't fall if you are inactive. As we are told in the FAQs, the ratings depend solely on how other members rate your posts.

I've had runs of inactivity and my rating has never dropped during those times. I highly doubt that being inactive will change your rating if at all.

I have been out for weeks on several different times because of being sick. My ratings have never dropped. Of course I'd come in a post a comment from time to time if I felt like it.

Ah jealousy by bad mylot members who ganged up with you with their friends and all gave you negatives. Also you may have answered a discussion that got deleted.

As far as ratings are based on voting there is no logic to lose rating if you stay inactive for several days. I believe it is so!