Perhaps your litter box is leaking? Is it big enough?
Maybe he doesn't quite make it in time... could he have a bladder infection?
A rabbit with EC might become increasingly incontinent - EC can be successfully treated if caught early (though never cured) - but I think vets can overlook it.
Is he neutered?
If you have some munching hay in one corner of his tray then chances are he'll already be in position when he needs to pee.