I have a Segger-Jlink and looking up on the pin out of the connecter, i found that there are 9 ground pins. Pins 4, 6, 8, 10, 12, 14, 16, 18, 20 are GND pins connected to GND in J-Link.
Why are there so many ground pins ?
All high speed signals get paired with a ground. You will notice the ground is every second pin. With a ground in between two signals on the ribbon cable you reduce cross talk. Parallel signals like on a ribbon cable cause cross-talk (unwanted coupling between signal paths). With a ground in between the primary coupling is to ground and signal level changes will not affect the other signals as much.
The other option to reduce cross talk is to reduce speed or cable length both of which will affect utility of the JTAG.
TDI, TCK and TDO is the high speed signals. This type of design is not ARM specific and you see similar implementations an any JTAG design.
The Electrical engineering Stack exchange will have more on cross talk.