USB ports differ as to how much current they can reliably supply at 5 VDC. It should never be assumed (without testing) that any given USB device can power your MixPre--and if other devices are also plugged in, they could reduce the available current at the MixPre's socket, depending.
There are standards as to the current that a USB port "shall" be able to supply, but they differ among USB versions 1.0, 1.1, 2.0, 3.0 etc., and the startup current is one thing while sustained availability is another. Plus there's a tolerance range around the 5 Volts, which may mean that in some cases the "5 Volts" you're getting from the port isn't enough to reach the "5 Volts" lower limit of the MixPre.