If the BUSPOWER pin is tied high (pulled up to 5V), the device will not be able to detect the presence of VBUS. So even when the hub device is not plugged into the host, the hub will have enabled the pull up on the D+ which is not legal. You must have the BUSPOWER pin connected to VBUS in order to detect the connect and disconnect event.
Please note that as per section 188.8.131.52 of the USB 2.0 specification, the voltage source on the pull-up resistor must be derived from or controlled by the power supplied on the USB cable such that when VBUS is removed, the pull-up resistor does not supply current on the data line to which it is attached.
Section 7.2.1 of the USB 2.0 specification states that devices may not provide power to the pull-up resistor on D+/D- unless VBUS is present (see Section 7.1.5). When VBUS is removed, the device must remove power from the D+/D- pull-up resistor within 10 seconds. So if the BUSPOWER pin is tied high (pulled up to 5V), the hub will consider the device to be in a connected state even though it is not plugged into the host. And the hub will have enabled the pull up on the D- which is not legal as explained above. You must have the BUSPOWER pin connected to VBUS in order to detect the connect and disconnect event and disable/enable the pull up on D+ accordingly.