All at once
Definition 1 of 2
Pro Tip 1/3Group Actions
Used when multiple actions or events occur together, giving a sense of unity or collective occurrence. 
The crowd stood up and cheered all at once.
Pro Tip 2/3Overwhelming Effect
Implies that dealing with several things simultaneously can be intense or overwhelming. 
Questions from the audience hit the speaker all at once.
Pro Tip 3/3Instant Coordination
Highlights perfect timing or coordination when multiple elements sync up exactly at the same point in time. 
The dancers started their routine all at once.