“Why is my daughter gaining so much of weight?”

It's often that children between 8yo to 11yo are prone to excess weigh gain. Because growth slows down and the body is preparing for puberty, sports and a smart diet help.

There are several causes, she deserves to be seen by an endocrinologist to have a full evaluation.

Obesity has become a pandemic issue globally. The body is storing excess calories and we are changing to be an obese society quickly. Most of our weight gain is due to excessive calorie intake and in activity. Some are genetic and few could be hormonal. Some people have a thrift gene that allows them to store calories more efficiently than others. But most stem from excess intake of calories and inactivity. Our diet is also highly refined and devoid of fibers.
Congenital, genetic abnormalities and certain medications also play some role in obesity.
The first step should be to discuss with you doctor about this issue. Have her proper evaluation to rule out, hormonal, genetic and other medical problems. If there are no medical problems, then structured physical activity, nutrition and behavioral modifications should be undertaken. Start it now when she is you. You will have better success.
If nothing works, there are other things to consider and you should work with your physician.
Good luck