SD card connector chassis grounding
They most likely have a ground for the micro SD card case and signals, and one for the rest of the circuit. It is common to tie shields to chassis ground to shunt ESD away from the rest of the circuit. 

Its a stitching capacitor its to ground the plains to provide a return current path across a slot or ground divide. If you run a trace across a ground divide then the return current has to go around the slot creating more inductance. The capacitor lets the high frequency portion of the signal jump the gap.



Unless you have a very large noise source, in my experience multiple grounds is a bad idea.  Get Electromagnetic compatibility engineering by Henry Ott