Except they also made it so he can jump up on the tower with the archers behind him if you try and just range him from there. So they obviously didn't just move him to that encounter with no changes.
That's a scripted action, it doesn't have anything to do with pathing or AI.
If the Taurus demon throwing himself off the bridge was thought out and intentional they probably would have given him an animation for falling off instead of just awkwardly dropping in his idle pose.
Having said that, there's plenty of places where enemies take themselves out in Dark Souls that are clearly unintended. I've seen serpent men run themselves off bridges in Sen's Fortress without any player input. I've seen drakes in the valley of the drakes fall into the ravine, and I've seen lightening demons in Anor Londo just back themselves off ledges despite the fact that they both have wings and can fly (which makes it extra silly). Painting guardians pretty routinely fall off the rafters trying to path to the player. Those are just the ones I can think of off the top of my head, and just in DS1. There's soo many more examples I could think of in the rest of the Souls series.
The AI in the Souls games is generally not very good, or very robust. For the most part enemies tend to do just run directly at the player to attack them, or occasionally have scripted sequences where they lead the player into a trap. This isn't really a dig at the games though. The majority of games don't have very good AI, and at least the Souls games tend to have the defense that most enemies are hollowed out walking husks and shouldn't be expected to be smart.
If you ever try speedrunning the games, or attempt to use weird challenge builds (like beating Dark Souls with just a bow for example) you would notice how bad the AI is at dealing with anything but the most common play styles.
