Two factors are at play,
1) It's a stressful and chaotic situation. The two people who were inside were told they have to get out now and Don said he'd see them on the surface... they might well have expected he was right behind them, rather than going on a time-consuming personal errand. Those on the outside were focused on the stability issues and assumed that Don would tell the others to get out and get out himself at the same time, either shortly before the inside team (after just yelling at them to get up there and once they acknowledged leaving immediately) or shortly after (if he was the type to stay behind and be the last one off the boat).
I mean, if you give someone this instruction:
Guy I Don't Remember The Name Of But Recognize From You Can't Do that On Television: West, I don't know how much longer these are going to hold. Go inside and tell Aiko we need to detach while we still can.
You don't really expect that they're going to stick around there much longer than necessary. I think it's safe to say that any individual person could have assumed Don came back when they weren't looking.
As these are scientists and leaders, in a normal circumstance, before detaching they'd almost certainly do a headcount and make sure everyone was clear, but...
2) The actual detachment came at an emergency situation, not dictated by choice.
Same Grown-Up You Can't Do That On Television kid: We're losing the Jupiter! Prepare to detach! We need to do it simultaneously.
Because, of course, the fear was that if they didn't detach at the same time and it went down, it would drag the other car along with it. If only one car was providing stability, they might well have detached immediately... or, they might have taken the time to make sure everyone was safe, but because the timing of the detachment was critical during the emergency situation, it probably diverted attention away and allowed people to miss Don.
If he was indeed missed by everyone. In a later ep we see many of these characters willing to give up on a critically wounded crewman for the greater good of the mission... it might well have been a similar situation, that some people knew he wasn't actually on the surface yet, but figured the situation was dire enough that they had to cut their losses and detach even if it risked Don's life (perhaps hoping that, if he was at least close enough to the exit, as any sane person would be rather than digging around a storage compartment, he might still have been able to get out when he felt it shifting or heard the order).
Judy obviously didn't know because she was surprised, but she was pretty focused on the engine readings, as was the other driver, who presumably just followed orders... but the others ordered the detaching to continue even though Judy shouted a 'no' just before, so one can assume they felt it was necessary.