Why is the community in The Giver ordered to go to the nearest building?
The community has developed its existence on the ideal of Sameness, meaning - among other things - that events are predictable and work according to their scheduled times and purposes. Unexpected occurences are highly upsetting because they do not fit the goal of Sameness that the community considers to be the best model for its life and functions.
Roughly a year before the beginning of the story told in The Giver, a jet had flown over the community twice at high speed. Because "it was against the rules for Pilots to fly over the community", this was an unprecedented event that caused anxiety, confusion, and fear among the community's residents, including Jonas. The citizens in the community had lost any ability to evaluate situations for themselves and were dependent upon their leaders for guidance in how to respond.
The first direction to deal with the unexplained event was the order issued through the public address system in the community.
Then all of the citizens had been ordered to go into the nearest building and stay there. IMMEDIATELY, the rasping voice through the speakers had said. LEAVE YOUR BICYCLES WHERE THEY ARE.