A transistor, usually, acts as a ... transistor! It is a device that the conductivity, between collector and emitter, depend on the bias of the base.
The circuit is designed like an asymmetrical class B amplifier. When the output has to be reduced (for whatever reason) the op-amp tries to do this by pulling current through R11 and R14 to the R13 that biases the Q2 and it "helps the situation"...