So how did a floor boss appear on floor 18? Was it an outside influence that forced a boss to appear? Or was it always there/brought by the goddess Hestias use of her godly power?
There are no monsters on Floor 18 however they can come from floors above or below Floor 18.
The question was answered on a subreddit-
From just the anime's perspective, the dungeon is perceived as a living being. It is also said earlier in ep. 10 or so that the dungeon hates gods. Right after Hestia channels that divine power, the dungeon starts to shake and the Regenerative Goliath drops down from what looks like the floor above. It appears to be a much stronger, powered up version of the Goliath that spawns on level 17, which just happens to be the floor above.
From just the anime, it would appear to be the dungeon reacting to the presence of gods by spawning a powered-up version of the Goliath and dropping it straight onto where it detected the gods.