Digital BW, The Print

I was printing something on my 2000P and I noticed that the image had
"ghosted" after printing; that is, it sort of faded out and a new,
offset image faded back in, with both overlapping each other.  The rest
of the print looked fine.  I assume that the paper moved or something
during printing, but how could it move in a way that would _overlap_ the
images?  What causes this?

I reprinted the image and it came out fine, but I'm curious as to what
happened.  I don't usually watch the printer while it is printing.