There's no way to know for sure what is causing your child to move a lot in their sleep.
But some children move around in their sleep due to bad dreams, regular, dreams, being restless etc.
It's normal for a child or even an adult to move around a lot in their sleep so there's nothing to worry about but if it's effecting your child's sleep and your child is waking up tired then you might want to take your child to the doctor and tell them about the excessive movement of your child during the night.
They might need to prescribe your child some sleep medicine to help them sleep better.
The movement of your child during sleep can also be caused by periodic limb movements which is very normal and nothing to be worried about.
I used to move a lot during my sleep as a kid and my son also moves around a lot during his sleep and he's okay.