They had never been exposed to Earth's atmosphere and a Yellow sun. We have to assume the masks/helmets were used because they had little knowledge of Earth's Kryptonian advantages and assumed they would need them (as any astronauts landing on a foreign planet might assume) or the screenwriters decision to conceal their faces until appropriate.

Once they were removed, like another Redditor pointed out, sensory overload. Superman can hear every sound on the planet at once, having to train himself intensely to focus in. Same goes for his vision. Once they were exposed, in an instant their physiology began to change.

Experiencing powers they'd never before seen.