In order to provide phantom power (usually 48V) for a microphone from the much lower voltage that the USB port provides, there will be what's called a DC-DC inverter. This will feed into quite a good sized capacitor to make sure there is no noise or ripple on the supply to the microphone.

In all likelihood what you are seeing when you plug the unit in with phantom on, is quite a surge when the inverter powers up feeding a discharged capacitor. When added to the switch-on surge from the other supply smoothing in the unit, makes Power Manager complain. I would see this as quite normal behaviour, although it has to be said, not a marvellous design. I wouldn't worry, as if there really was too much current being drawn at any time, rest assured Power Manager would tell you about it.

To be honest, I like to plug these sort of devices in and then power-up the Mac.