It seems as though my pi zero does not boot or send image through HDMI until I attach the USB hub. Why is this and can this be fixed?
Yeah, this could be because they power supply you are using is not supplying enough power. Do you have your default power supply (Should've came with the Pi)? The Pi requires 5 volts for it to run, and so your battery pack may not supply enough voltage. You can check you battery pack for a little sticker or something saying the input voltage (going in) and output voltage (out to the Pi). On the original adapter i know it is 5 volts.
Also, what USB? An additional power source? A storage device? If it is power, it will probably only boot because it has enough power to rather than you just trying to boot it with your one battery.
I hope this helps, Dallin