Honestly, probably all of the above. I suspect that the screen lifting did allow the phone to become water damaged where otherwise it might not have, but at the same time people taking their phones into the bathroom with them while showering is a major cause of liquid damage. Even with direct contact with water, think about all the steam in the air.
Unfortunately, most repair shops will tell you that they warranty your screen for the lifting because of the liquid damage. Understably, customers often get pretty upset by this response, but the truth is that shops dont say that just to be mean. You would be amazed the number of people who will cook up a scheme to con repair shops into covering the cost of repairing a device that the customer actually broke prior to ever coming in or broke again after leaving. The only way to prevent this is to insist that customers immediately return the device to the store if it needs a warrantied repair or risk not being covered if the device or part is further in the meantime