I have long been of the belief / recollection that Talkback is primarily responsible for MoK at kick off, because you're right, it's not something we've always done. I don't know how many years ago it was, but Geoff (the old Forest webmaster) took a straw poll on Talkback as to what song Forest should play just before kick off - I forget what the other options were, but MoK was definitely the clear favourite and the week after, it became so and has stuck and grown ever since.
Obviously Geoff was active on other, busier Forest forums and he may well have asked the same question there, but I am not and so I choose to believe it was entirely our doing.