Plugging iPad into computer turns on Personal Hotspot with one connection I have an iPad 3 with personal hotspot turned on. I also have a Macbook Air connected to my local wireless network (the Air is not connected to the iPad personal hotspot). However, when I plug the iPad into my Macbook via USB, the iPad personal hotspot turns on and I see the blue bar at the top with one connection.  Can you explain why this is happening?  

Did the Macbook stop using my local network wifi internet connection and start using the iPad's personal hotspot connection? Is this using up my cellular data allowance while I use the Macbook when the iPad is connected to the USB port?
If you plug any cellular enabled iOS device with Personal Hotspot turned on to a Mac via USB, the Mac will automatically open a network connection using the Personal Hotspot.

So yes, your Macbook Air will be the 1 connection you see.

The second part of your question is a little trickier and the answer is both complex and confusing but I will try to explain as best I can.

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Based on older knowledge with other multiple interface *nix systems is that it will be a complete toss up as to which connection your Mac will use since both active connections could possible provide the same route to the destination for any network query.

I have even seen, on both Windows and Unix networks, traffic coming in on one network connection and going out on another, which rarely works well.

To get round this, in the Network panel you could disable the "iPad USB" in your default location, but then set up a new location where your "iPad USB" is not disabled.

Finally, the order of your devices should mean that if your wifi is above the iPad USB device in your list when both are enabled, it will get priority over the hotspot, but there are still caveats and this could still use cellular data, but of a lesser amount.