Why at the end of the rally did everyone from the MInistry of Truth have to go to work?

It was party policy that everyone return to their posts. A rally is simply one part of the duty of a party member,

"The instant that the demonstration was over he went straight to the Ministry of Truth, though the time was now nearly twenty-three hours. The entire staff of the Ministry had done likewise. The orders already issuing from the telescreen, recalling them to their posts, were hardly necessary."

A rally was not an excuse to simply do something else once it is over.

Oh, they also have a hell of a lot of information to change now that Oceania was now at war with Eastasia and not Eurasia.