Ethernet driver Circuit
This circuit is known as Bob-Smith termination. It is used to enhance EMI and ESD performance of the Ethernet circuit.
The unused pairs are connected together through a 75Ω impedance matching circuit to chassis ground through a 0.001uF, 2 kV capacitor.The capacitor provides a discharge path for noise immunity on the unused pairs.

Also,termination for the media-side centre taps is comprised of individual capacitors(C3) to chassis ground.