I think you've hit the nail on the head, Scott. I'm sure I've done it, too. Something in the post or a previous response just strikes a cord, and we veer a bit off course. It's no different than a face-to-face conversation. My suggestion to the original poster would be that if you see this happening, post a gentle reminder in the thread to clarify your question or point, just as you'd attempt to get a conversation back on track.
As Scott said, I don't think this is done intentionally.
Wish that I could cry
Fall upon my knees
Find a way to lie
About a home Iíll never see
It may sound absurd...but donít be naive
Even heroes have the right to bleed
I may be disturbed...but wonít you concede
Even heroes have the right to dream
Itís not easy to be me