How it's supposed to work:

1. Passengers exit the train
2. People board the train
3. Announcement that doors are closing
4. Doors close
5. Train departs
6. Everyone is happy

What sometimes (rarely) happens

1. Announcement is made whilst people are still exiting the train
2. Doors close whilst people (me) are boarding

The problem with (1) is that it causes people who are waiting to board to try to push past the people who are exiting the train. Which obviously causes delays, but if the station staff are not checking carefully (which is my suspicion about why this is happening) they then close the doors before everyone is onboard.  Which makes me unhappy.

And, yes, it would be terrific if everyone did wait for people to exit before they try to board, but frankly the risk of the doors closing before you have boarded make this a bit more excusable.

[In case it's not clear, the MTR is generally excellent, which is why the occasional problem is noteworthy]