This lovely natural lightning rod.
That part is the bald, top half-mile of the mountain, and it gets hit by lightning all the goddamned time. If you look closely, you'll notice that the people in the photo are climbing up a ladder made of steel cables, which in Yosemite (and much of the rest of the world) are known as lightning rods, and incoming rain will make things even worse. Yosemite reports Half Dome getting struck by lightning at least once a month, or "way too fucking often for you to camp up there."
About midway through Bruce's hike, the weather got shitty. One of the members of the group assured everyone that if things got bad, they could take shelter in a cave at the top of the mountain. Here's that cave:
Bruce Wayne's cave is a sanctuary; Bruce Weiner's is a deathtrap.
How much protection did that offer? Let's put it this way: Not everyone in the group survived what was about to happen.