From the article - and this could explain how it happened"

"Rescue crews found his camp, where he still had several pouches filled with water, that Sunday."

and further on:

"Police suspect Tanner may have been hiking to or from Clapp Spring, which is described on the Bureau of Land Management's Web site as a palm oasis and permanent water source for wildlife. Locals say Clapp Spring is little more than a muddy patch that supports a few palm trees but does not have enough water to drink."

So Tanner may have been acting on bad information. He left his encampment, leaving behind some of his water, as he headed to what he believed to be an "Oasis" to get more water. Finding little water there, he tried to get back to camp but didn't make it due to dehydration.