Why would an airplane tip backwards during unloading?
A picture is worth a thousand words... Passenger airplanes do not fasten baggage in the belly down, occasionally there can be a concern from the baggage pilling up at the tail (i.e. turbulence). New baggage employees may also have offloaded the aircraft wrong - unloading the front first. 

I have been on airplanes where the flight attendants moved passengers from the rear of the airplane to first class because the aircraft was too tail heavy.

Most newer jets have pressure sensors on the gear. This allows the flight computer to show the crew how the aircraft is loaded in real time. The pilot may have received a warning the nose gear was too light or he may have notice the aircraft tipping up.