Fairly simple to isolate a door on a 29k, worst case you lock the coach out, taking the entire train out of service is a bit of an over reaction, however there are unconfirmed reports of other damage
Don't really buy the door issues, as you say the obvious solution and done time and time again. Max 15 minute delay.
What I have seen do the rounds and more believable is a fight or some sort broke out on the 2900 and of course guards took there time to arrive hence the delay and all the disruption.
Now transport police would have solved the problem.