OP, I wouldn't worry.

It's happened to me too recently. All my stars have been rated down and I also have no idea why.

I post either the same day, or the next. Items are packaged well. I also include a thank you note.

My item descriptions are thorough without being over-bearing. A lot of buyers don't like reading an essay so I keep that in mind.

My communication is also fast, efficient and polite.

My postal charges are always only what AP charge me, even sometimes less.

Yet, some buyers aren't happy. I wouldn't worry yourself. Your feedback looks great.