I can understand nobody replying if nobody knows the answer.
As for the second phenomenon, I think people are waiting to say something in particular, and when a topic even remotely related pops up, they see it as a chance. Others just like to post.
My mother was an example of both. Talked all the time. If you said something, she would find a way to steer the conversation back to her. For example, if you said you broke your leg, she'd say, "yeah, you know, my ankle has been really sore lately". Not really a criticism of her, it's just the way she was, and I think there's a bit of that in all of us.