PLEASE, BE AFRAID !  I have also taken  used rags and  opened them up flat
and layed them on the concrete or outside with a rock on them so they cant
blow away. I am not a chemist so I cant give the exact reason that these
rags "Spontaneously Combust " . I think that it happens when the used rags
are wadded up and put in an inclosed container ( bucket , box,ect. ) with
the presence of oxygen. If im doing a project where I know I will be
applying more coats of oil for a few days, I have put them in a Ziplock and
squeezed all the air out, This Still scares me,so I am careful to keep a
close watch on the bags.   I think it works something like a compost pile
that builds up heat in the center. EXOTHERMIC ??  Please Respect oil soaked
rags.   Happy Thanksgiving everyone.