This can happen if the child is not sitting sufficiently deep in the carrier.
This is because the booster seat should be inserted with the fabric pointing downwards towards the waistband, so that the label "Sit baby here" is on the widest part of the booster seat. This is where your child's bottom will rest.
In addition, the apron of the baby carrier should also be against your child's bottom. In the first photo below, only half the thighs are resting. In the second photo, the buttocks and thighs are fully supported. The weight is tilted backwards and is thus better distributed, lightening the joints.