It's been years since I've seen a similar incident, at Hatfield, where the driver closed the doors as people were still boarding (I believe it was in the Wagn days!) and it wasn't that the train was crush loaded or anything, there had simply been a lot more people than usual alighting through some of the doors, meaning those on the platform had to wait.
It seems to me rather crazy for a driver of a train that's only 2tph to not bother to let people on simply as so many people got off - and then go off half empty, leaving people to wait 30 minutes for the next train, for the sake of a few seconds. Imagine if they did this at Finsbury Park!!
As it was platform 1, the driver could look out of the window or use the mirrors (as I believe there were then, CCTV being added a lot later) and could only see that there were large crowds by the door. Is it safe to just press the door close button, without even saying anything (okay, maybe the PA system didn't work - who knows!).
Clearly given the recent incidents on FCC, drivers must be a little concerned when they close the doors as people are still trying to get on (when it isn't a case of someone rushing up last minute and trying to hold the doors open). How can a driver clearly see that they're not going to cause an injury? In the above case the train was a 317/1; how could the driver be absolutely sure the doors weren't going to trap an item of clothing in them and drag someone along?
I'll never know why the driver didn't wait a little longer for the doors to become clear - as they would have done with another 10-20 seconds of waiting, or use the PA to tell people to move away, but thankfully I've never seen anything like it since.
Sadly, from what the OP has said, it has happened again.