You and the hashiras were fighting a demon, but he used his demonic technique and sent you to another dimension in this case Danganronpa V1, for some reason they did not carry their katanas...

Sayaka: It's a pleasure, I'm Sayaka.

Mokuro: I didn't think we were going to see more students.

Monokuma: Don't continue, I want to see who the new guys are, but in 15 minutes everyone will be here.

The hashiras and Tanjiro's squad didn't know what was happening