Boulder Mechanism in Sen's Fortress

First time I went through Sen's, once I rotated the boulder, it never changed direction again.

But since then, everytime I go there, if I don't stay in the room, the mechanism keeps spinning. Is this meant to happen? Does this have any relation with the giant on top in the roof?

While I can't really comment on why this happens, I have to concur with the scenario you've outlined.

My first time, it kept spitting boulders out of the hole in the wall once I had moved it to that position. On the subsequent play through, I'd moved the mechanism, died, and ran back through. Assuming boulders posed no threat, I ran carefree through the fortress only to yell "WTF?" (the actual words, not the acronym) when a boulder flattened my character with a vengence.

I would imagine that this is just a strange anomaly. Perhaps connected to one of the patches.

It will usually rotate itself when you cross certain triggers. When you approach the first chute (the one that's outdoors), for instance, the mechanism will rotate itself so the boulders are in your way. Then, once you've entered the second room after the chute (rooms on the left, midway up the chute), it'll rotate itself again so that it's on the next chute (the one that's indoors; you can choose to go up or down this one). If you go down instead of up, on this chute, then go up the elevator and arrive at the chute that rolls boulders in front of Logan, it won't rotate itself there. Any time you cross the first two triggers for the first two chutes, though, it should rotate itself.

Also, anytime you're on the second chute (especially when you're going up; not always when you go down), the mechanism will rotate itself to shoot boulders at you (like when you're a phantom and you rotate the mechanism for the host so he can come up, the mechanism will continuously keep rotating itself toward the host).

Sometimes it won't rotate itself, I don't know why. Most of the time, though, it will rotate itself automatically in those two places.

It's especially useful to know the boulder mechanism triggers when you're invading. If the host is ahead of you, and the boulders are on the first chute, you know he most likely hasn't passed the second room. So he's either in the first room off the chute, or in the one at the top of the chute where there's a manserpent-mage, where you get the defense ring. If you're at the bottom of that chute and it won't rotate, it's most likely because the host is waiting to ambush you in the first room.

The Forum Hunk is correct.

After moving it I've never had it move back to the position where it rolls them down the long chute outside, the first place you encounter them. But after every death it always resets itself so I have to deal with them on the stairway.

Welp, that was quite an explanation Blaine. Nice to know the mechanism reacts to triggers and it's just not just the game's hate towards the player lashing out.