1) "apparently it was just some silliness conjured up by The Basil Brush Show, a British television program that started in the late 60s. From there, the Australians turned the original expression “don’t get your knickers in a twist” into “don’t get your knickers in a knot” "
Source:
http://www.theblog.ca/


2) "Try putting your knickers on very, very quickly - say you need to put them on before someone walks into the room you are in. That is probably how you'd twist them. You might even have to abandon your efforts and hide them in your pocket - not that I'm speaking from experience."
Source:
http://www.phrases.org.uk/.../1268.html