My understanding is that the human eye has pain receptors that respond to wavelengths in the visible spectrum. As the moon occludes the sun, the level of visible light drops below the level where prolonged viewing causes pain.

But visible light is not the only frequency. Sunlight also contains UV and IR wavelengths. The human eye does not have pain receptors that respond to these frequencies.

The end result is that during an eclipse it becomes more likely that a person will fries their eyes due to the presence of UV and IR wavelengths.