Rather than being an offshoot, it is my understanding that Matsumoto Bizen-no-kami-ki no Masamoto is believed to have studied with Iizasa Yamashiro-no-kami Choisai, the founder of Katori Shintoryu, among others - but it is very much it's own art with it's own creation myth, founded in 1450.
Check out "Legacies of the Sword", by Dr. Karl Friday - outstanding book.
Also:
http://www.kashima-shinryu.us/ and http://www.kashima-shinryu.jp/English/index.html