When you add a USB device connection from an ESXi host to a virtual machine, the autoconnect feature is enabled for the device connection. It is not disabled until you remove the device connection from the virtual machine.
With autoconnect enabled, the device connection re-establishes in the following cases:
The virtual machine is cycling through power operations, such as Power Off/Power On, Reset, Pause/Resume.
The device is unplugged from the host then plugged back in to the same USB port.
The device is power cycled but has not changed its physical connection path.
The device is mutating identity during usage.
The USB passthrough autoconnect feature identifies the device by using the USB path of the device on the host. It uses the physical topology and port location, rather than the device identity. This feature can seem confusing if you expect the autoconnect feature to match the connection target by device ID.
If the same device is plugged back in to the host through a different USB port, it cannot re-establish connection with the virtual machine. If you unplug the device from the host and plug in a different device to the same USB path, the new device appears and is connected to the virtual machine by the autoconnect feature that the previous device connection enabled.
Autoconnect is useful in cases where devices mutate during usage. For example, for iPhones and other such devices, the device VID:PID changes during software or firmware upgrades. The upgrade process disconnects and reconnects the devices to the USB port.
The USB port is speed-specific. The autoconnect feature assumes that devices do not transition from USB 1.1 (low-full speed) to USB 2.0 (high speed) or the reverse. You cannot interchange USB 2.0 high-speed devices with USB 1.1 devices. For example, you might connect a USB 2.0 high-speed device to a port and connect that device to the virtual machine. If you unplug the device from the host and plug a USB 1.1 device into the same port, the device does not connect to the virtual machine.
For a list of supported USB devices for passthrough from an ESXi host to a virtual machine, see the VMware knowledge base article at http://kb.vmware.com/kb/1021345.