It isn't "having just stepped on the floor". Close enough for the sound to carry, but MC & co. had to go through some passages and encounters to actually get there.
Not many panels spent on that, but they are there...
Actually, Frayne's group had a lousy luck: Wade's group made it to the fifth floor first, chatting happily, but then turned around and returned to the fourth floor. Frayne's group entered the fifth floor immediately afterwards. So, Wade's group would have basically alerted/attracted any monsters within sensing distance to the location of the stairs to the fourth floor. Thus those monsters would have been waiting for Frayne's group when they arrived.
That giant chimera must be sorely lacking prey, so it would be all too happy to react to any sign of adventurers. Since there probably are multiple stairs between the fourth and fifth floors, it can't just permanently camp in a single location, lest adventurers might entirely avoid it. Logic dictates that it either wanders around, checking all the stairs periodically, or it has underlings monitoring the stairs.
