Why are USB power and ground connected at the same time?
SATA drives typically have higher current concerns than usb does. Power hungry motors in hard drives can lead to large inrush currents, causing draining and spiking on the voltage rail. 


  Drive power connector pins 3, 7, and 13 are longer than the others, so they make contact next. The drive uses them to charge its internal bypass capacitors through current-limiting resistances. Finally, the remaining power pins make contact, bypassing the resistances and providing a low-impedance source of each voltage. This two-step mating process avoids glitches to other loads and possible arcing or erosion of the SATA power connector contacts.


Usb on the other hand, has lower current sourcing (by standards limited to 500 mA), and is hampered by backwards compatibility constraints. The plug was designed way before USB became a common power source.