I can't imagine any reason for the underside of the tarp getting that wet unless the tarp was not made of the right material. Could it be that somehow Clark got hold of some fabric that was not properly manufactured?
The weathershield is only 'resistant' to water and will let moisture through if enough is present. But still, moisture dripping from the tarp should have mostly run off the weathershield.
Any chance that you were sabotaged by your camping partners?
Perhaps a phone call to Clark will help.