Change to Siegmeyer?

In Sen's Fortress, he is located below the boulder trap past some snakemen corpses. That wasn't so the case in my game: Siegmeyer was indeed there, but he was being guarded by snakemen, in addition, his dialogue (IIRC) about the boulder trap comes up the first time you speak to him and not the second.

Is this just some buggy bug or an actual change to the game?

well, sometimes those snakemen are dead and sometimes they are alive. I'm not exactly sure what decides that. Not sure about the dialogue, probably has something to do with that. I started NG+ with a character today, and was surprised to see the snakemen dead, they are usually alive the first time through sens for me.

I had this problem once a few months ago. I actually came in to find him gone, and not only the snakemen around that area dead, but the majority of the ones immediately following that area as well. There's a thread here somewhere where I posted it, but I can't remember what we decided caused it. At any rate you should be able to continue his storyline normally from Anor Londo. Pretty sure I did.

The intended event, is that the serpent-men around him are dead, because it's supposed he killed them to get there in the first place, and he sat there to think how to go up.

But yeah I had that bug that causes them to be alive, be careful with Siegmeyer, once while I was fighting them they it Siegmeyer and almost killed him, and it's very probable that if they stagger him he will fall and die, because he is a sitting on a cliff.

I've had it happen both ways a few times each. Just another hint that the world we think we know still has some mystery.