Thank you for posting!
Your question has helped with your answer: "I have not been plucking them and they are just naturally getting thinner. What are some possible causes?"
Waxing and plucking can cause hair growth to diminish rapidly but there are other reasons such as alopecia, immune disorders or hormone issues. As you say, your eyebrows are 'naturally' getting thinner. And this is due to one thing - simple aging. It can happen as early as the 30s and right up to old age. Even body hair (legs, chest, underarm, pubic hair) can get a lot lighter for women as the years go by.
For some people, especially men, their eyebrows and ear hair (and nose hair) can get very thick, with age. For women, the opposite can happen.
Talk to your doctor about the changes you are seeing. In the meantime, if you want your brows to look thicker, use a brow pencil or brush with eyeshadow, to fill in the gaps.