What is the purpose of a capacitor to ground?
A capacitor's complex impedance is given by

$$
\frac{1}{j \omega C} \Omega
$$

where \$\omega\$ is the frequency, and \$C\$ the capacitance. For a low-frequency signal, you can see that the impedance will be relatively high. Conversely, you can see that for a high frequency signal, the impedance will be quite low.

A high-frequency signal will see the capacitor connected to ground, and travel through it, since it is a low impedance path, but a low frequency signal will not be affected by it. The capacitors to ground form a low-pass filter for the lines they're connected to, as they remove high-frequency signals from the line by giving those signals a low-impedance path to GND.

See this question.