^^This.
"I am the ultimate badass, you do not wanna *beep* wit' me!" Hudson in Aliens.

Even if they were blocked from "view" at the moment, if the shreikers happened to wander past the solid wall, they would discover the people. They couldn't have stayed there long and I think that is basically what Bert was thinking when he panicked.

well they are still learning aren't they?
but dont be too quick to judge here, he was out of breath at the time and was the only person who saw that they stopped under the door so that could also be a reason. So when they realise this, Burt and Grady grab a door and "step and step" away
you just chose a stupid moment to pick on.
Try listening to when the Shreikers when they scream can you hear human voices? (and I don't mean the moment when Grady is "BLAAAAAAAAGH!" ing at them on the oil tower).