Which OS is the computer running? Some OS'es give a notification of an over current situation and automatically switches off the power to the USB ports or sometimes forces shutdown of the computer all together. The (OC) pin on U4 stands for "Over Current" and when the temperature of U4 is past a set limit, pin 5 gets "pulled low."
Does U4 feel abnormally warm after shutting down?
"When the output load exceeds the current-limit threshold or a short is present, the device limits the output current
to a safe level by switching into a constant-current mode, pulling the overcurrent (OCx) logic output low. When
continuous heavy overloads and short-circuits increase the power dissipation in the switch, causing the junction
temperature to rise, a thermal protection circuit shuts off the switch to prevent damage. Recovery from a thermal
shutdown is automatic once the device has cooled sufficiently. Internal circuitry ensures that the switch remains
off until valid input voltage is present. This power-distribution switch is designed to set current limit at 1.5 A
typically." -
TPS2061 datasheet