The brilliance of the light. Its like the sun exploding. I have nuclear flash goggles.
If the bomb burst close to the ground and nobody expected that to happen, then that would be understandable. They got a burst of light at a low angle when all precautions were against a burst of light overhead. That must have been a “DAYUMN!” moment.
BTW: Welding glasses will make good eye protection against a nuclear explosion.