Trying to understand negative voltage generation with capacitors and diodes
The only path for C1 to discharge is for current to flow through D2, which applies a negative charge to C2.  If you're having difficulty with this, first visualise what is happening to C1 in isolation - it's charged to 9.3V and the positive plate is then connected to ground, so the negative plate will be at -9.3V.  Once you're happy with that, see how the rest of the circuit will behave.