I've noticed similar problems on certain forum threads. And that's individual threads, not entire forums, so it's not a bug in the forum software.
I discovered at least one cause of the problem, although I have no idea why it causes a problem: linked flash files. In threads where someone posts a link to a .swf file, Safari almost always crashes, but only when I begin to scroll the page. What's even stranger, so does iCab! Albeit not as consistently. I'm pretty sure it's the flash link, because when I disable plugins in Safari, it runs just fine. Is Safari doing some kind of pre-fetching (and pre-running?!) of linked files? And if so, is there any way to turn it off? The flash file itself doesn't cause Safari any problems.
Might this be the same problem you're having?