Don't know the cause. I'm finding that RIPS for digital printers can be a crap shoot as to whether you get one that is quarky or not when printing (from photoshop).
I may be wrong, as I don't know about printing cmyk seps (straight out of the cmyk conversion from RGB), but from what I do know that works, you need to have each CMYK channel (as a spot color). You can try copy and paste (then invert) depending on the channel mode...then paste into a new channel. Name it as the color from what you copied from and try printing.
As a work around, try breaking the channels up into individual files per color. (SPLIT CHANNELS) Then print as a single grayscale image (or just print).
That SPLIT channels may not work on straight CMYK from the CMYK channel sets. again, I don't do it that way.