Hi David, thanks for watching. It could be a lot of different things unfortunately and hard to know without being there to see it. Sorry I can't be of more help and hope you get it fixed.
It can be caused by a pipe to or from the valve that is leaking. It pushes water along between the pipe and the soil and fills up the box. The valve could possibly have a bad gasket or be cracked, perhaps from frost. My ones have a leaking sprinkler at the top of a slope and the water runs down hill and into the box, filling it up.
