Thanks for the replies, all.  I was operating FT8 on the wsjt-x standard frequency, 14.074, and I'm  aware that 14.0715 is a standard PSK slot (indeed there was one PSK station in the mix).  I often do PSK there.  But PSK timing doesn't follow the :00, :015, 0:30 etc timing of FT8.  The funny stuff I was seeing did.  Theories about sidebands etc. don't explain why there were several stripes in the waterfall (one station maybe, but...).  Anyway, it only happens occasionally, and causes no real trouble.  But I'd love to know...  73, KD9PCN