How does this circuit automatically oscillate at the microblower's resonant frequency?
I assume the "microblower" (what is that?) is connected via the J1 connector?  If so, then the the microblower is series resonant at it's operating frequency, and is providing positive feedback to the amplifier at that frequency.  I assume that there is little or no DC continuity through the microblower.  

The RC circuit (R7, C6) is providing negative feedback to stabilize the amplifier operating point.  C6 lowers the gain at high frequencies to keep the amplifier from self-oscillating when the microblower is not connected.  When it is connected, the positive feedback overwhelms the negative feedback, and the circuit oscillates.