USB power is not enough for servos - the computer USB hardware will be detecting over-current draw on the USB power and disconnecting automatically, the software drivers note this error condition and probably retry to establish the link some short time later.
Assume every servo can pull spike currents of a few amps and design accordingly. Almost always a bad idea to share power to a servo or motor with a microcontroller. In fact ~90% of posting on servos here are because of this.