How to connect arduino with shield correctly?
It sounds like you might have a bad solder joint. Without anything else connected I would check all of the traces from your usb host shield to each header pin. If those all check out try the same on the uno. You may also just want to consider doing a re-solder (on the header pins of the shield) and try again.

When you are applying pressure it may be moving the pins slightly and making contact.

Also make sure the shield is completely seated in the uno headers and that the UNO powers on w/o the shield attached.