Here's an interesting article on how to forge headers

http://www.cynthiaarmistead.com/headers/headers.shtml

In particular, this part: "I know that some things can be forged, but
haven't a clue as to how to forge them--it isn't something most people ever
bother to learn, even if they are heavy internet users. Only net abusers and
those who want to stop them have to go to such lengths. "

I'll give you a scenario...

Someone starts posting on-topic for an extended period; might be a couple of
months. Then, they start posting things that are either off-topic or in a
quantity that is intended to disrupt a newsgroup, even if they say ahead of
time that they are going to do it.

Someone else (sometimes many regulars and lurkers) says "don't do it,
please", and that person (people) is then targetted by a couple of the OP's
friends, usually using forged header information in an attempt to hide their
real identity.

People have come into this group, stating that their particular method is
superior to that which the group has preferred for a long time. They have
claimed special knowledge of the subject, in this case newsreaders, but it's
recently happened with yEnc as well.