I believe this is more trivial
Could it be that, it's derived from the Japanese name of Pentax or Ashahi . Their a lot of K in the historical name
Asahi Optical Co., Ltd. (旭光学工業株式会社,
Asahi Kōgaku Kōgyō Kabushiki-gaishaand when the company was rename in 2002 took the name of Pentax Corporation
Pentakkusu Kabushiki-gaisha, and Pentax K mount is really PK mount
You can read a lot in Wikipedia on K mount but they dont talk about that fact
K seem to be a very popular letter it seem in Japan
OR Could it be because when you install a lense it's about the angle between the position 0 and the locking position who is about angle on the K
Or The K Bayonnet system is base on three tab like a K
Also The K mount was co invented with the German Zeiss and the German use a lot of K
But hitorian can give us the correct answer