Looks like it was quite low light in the room. If the light levels are low, the phone will take a longer exposure photo to make it brighter. She was probably moving her leg quite quickly and because the longer exposure photo, her moving leg will blur
I don't know if you have tried Pro mode But you can control the camera settings yourself So normally if I'm in a low light environment. I will open the camera pro mode and adjust the shutter speed a bit quicker and increase the ISO That way if a photo opportunity arises, I have settings ready. Now it depends on what the light level is to what settings you use
It's also worth trying if you don't want to do that. Take multiple photos in one go. Then hopefully at least one will be good
