When you have common collector, and inductance on the emitter, the base has an apparent negative resistance. Just add LC tank, and you have yourself an oscillator.
Usually around 20 ohms in the base or collector, at frequencies where this is relevant (i.e., 1 to 100 times below fT), dampens the negative resistance. A ferrite bead does a good job, having a largely resistive characteristic at RF.
Tim