Once upon a time there were few computers and no powerpoint slides. If you were in the Agriculture Extension, you carried around a bunch of ink stamps to illustrate crop rotation and such to your farmer clients.
If you were publishing a kind of "underground" book about photography in 1973 (titled, Photography: Source & Resource) and you were looking for a logo for your (self-published) publisher, you might like the Turnip stamp. Thus was born Turnip Press (which never published another book, alas). The Ag Extension teacher was the father of my co-author, Dave Tait. We use the turnip in memory of Dave.
So when we went to find a name, the Turnip was handy. And it's really really simple. It's organic. The best part is underground. It's totally not a techie, digital, slick, electronic kind of thing so it seemed right for us.
(The Turnip is great unless you're French, in which case "navet" was probably not a good choice for us.)