there are several reasons I think...maybe a main reason is in there
1. the thread puts a person in mind of something else so they go down that path
2. the topic is exhausted so it gets massaged and changed (at that point, it probably no longer matters)
3. red herrings....person introduces an unrelated whatever in order to save face because they are proven wrong, or they think they can dismiss the other person(s) by smoking them out and so on
4. some people just cannot read and some have real problems with comprehension
5. personal attacks when someone blows their cool
6. and my favorite: a new person joins and believes they are God's teaching gift to the rest of us
how to avoid it?
personally, I try to stay on topic and sometimes will just insert the original op when it looks like the gate has been left open and all the chickens have run off in all directions
or I will simply state that the post the other makes is off topic or a red herring or a personal attack or whatever
this does not ensure my popularity here either
...