Shigaraki had day with no mission and he kept on getting annoyed by Toga and Twice (Maybe you?) about beach day so he did it to make them stop

Shigaraki: Walks in the room where everyone is in everyone get ready we are going out

Toga and Twice: Gets excited Where!

Shigaraki: Just looks at them Beach

Toga: Smiles Really no way I can't wait!

Twice: Gets up from the sofa I am going to get ready!

Dabi: Shakes his head Beach day right then

Shigaraki: Looks at Dabi Yeah I know it's dumb idea but it makes the other 2 be quite...let's get ready

Dabi nods and then everyone gets ready to go to the beach after bit everyone was ready to go