Or - There is a bomb on one those 2 aircraft that just taxied out.
* Do not tell them or I will blow it up.
* If any attempt is made for people to get off I will blow it up.
* Make what ever demand they want.
So if the Captain refuses to follow an instruction and just to get an answer over radio decides to evacuate - BANG!
There was no know reason for him to know of any possible danger that would require an evacuation - they would have had food water and air conditioning.
If you don't have all the information it is foolish to make a non standard decision - and an aircraft evacuation often leads to injury's.