Yes hormones again! Oestrogen plays a crucial role for women in maintaining bone density- it reduces the numbers and activity levels of the cells that lead to bone being broken down (osteoclasts) and keeps them under control. When oestrogen levels fall around the menopause these cells become much more active and break the bone down quicker than it can be reformed.
The rate of bone loss for women at this time is about 2-3% per year for the 5ish years around the menopause (though this varies hugely from woman to woman) with some women having a higher rate of bone loss for longer.
In a cruel twist of fate we know that women don’t achieve the same peak bone mass as men AND we lose bone rapidly at the menopause- so it’s a double whammy for female bones which is why osteoporosis is much more common in women than men.
Of course, the higher your peak bone mass in your twenties and the less bone you lose before the menopause accelerates bone thinning, the lower the overall impact of the rapid bone loss around the menopause will be.
This goes some way towards explaining why it’s so crucial to be aware of your bones as early as possible.