The simple answer: The hijacker exchange your ID with his affiliate ID.
Two links:
http://www.websitemarketingplan.com/...iateHijack.htm
http://www.associateprograms.com/sea...etter205.shtml
If that does not answer your question, make a search on the keywords.
Kjell Gunnar Bleivik
http://multifinanceit.com/
http://www.blognorway.com/