They had seen the people running past without looking back, though they hadn't yet identified the threat, but both heroes were somewhat surprised to see each other—perhaps Dynamight was a little more annoyed than surprised...

“What are you doing here, Deku!?” He's asked by the explosion user, with evident confusion. “I was sent here! But... I'm not sure about what might've happened...” Deku looks at Dynamight like he expects some sort of explanation from him; any information would be useful, but the other hero was in the dark as much as he was.

The only thing they both knew for sure was that a disaster was going on. The duo has no idea what they were about to deal with... But it was probably a good idea to work together.