Just read the IC.

Kevin saw James, so he left his room but ended up seeing Gwyn and went after her. Immediately after, Stewart came out of hiding just to see Gwyn running downstairs and then Kevin following. So he goes down, just so Donny can come down to see all three of them bunched up on the stairs. Oh yeah, and the fact that Gwyn made that much noise going down, if walkers came from both directions then they are fucked xD