Very common. 1. Jaws grow slightly unevenly.
As they do, kids tend to "grind in" their occlusion so teeth mesh better. 2. Kids back up the "computer" @ nite. They learn so much during the day, they tend to process it when they sleep
. Grinding should disappear as permanent teeth start to erupt. By the way, 1st orthodontic cm-up age 7.