Just speculating on my part, but for fun follow this path and ignore the other components:
NPN collector, up the 18k resistor, stop. Over to one side is a .022 cap to ground. Turn left and head down the 560k resistor, stop. Hey, another .02 cap to ground! Continue heading toward the NPN's base along the next 560k resistor, then stop at the base. With the input control grounded, why look! There's another .02 cap to ground! We got ourselves a phase-shift oscillator!
IIRC gain has to be above 29 for stable oscillations. Can you estimate maximun hfe to be below that limit?