2 main reasons
One reason is that jaws grow unevenly, a little here, a little there.
Kids "grind-in" their occlusions to accommodate. Second reason is they learn so much during the day, they back up the "computer" @ night. Grinding should diminish to disappear as the permanent teeth start to erupt. If occlusion is a problem, remember 1st orthodontic ck-up age 7.