Which theory was most likely to have caused the Tacoma Narrow Bridge in Washington to collapse in 1940?

On November 7, 1940, at approximately 11:00 AM, the first Tacoma Narrows suspension bridge collapsed due to wind-induced vibrations.

The Tacoma Narrows Bridge was known for its tendency to sway in windstorms. The shape of the bridge was much like that of an aircraft wing and under windy conditions it would generate sufficient lift and become unstable. On November 7, 1940 a large windstorm caused severe and catastrophic failure if the bridge deck.
The significance of the first Tacoma Narrows Bridge, to a large extent, is derived from its startling collapse on November 7, 1940. The collapse brought engineers world-wide to the realization that aerodynamic phenomena in suspension bridges were not adequately understood in the profession nor had they been addressed in this design. New research was necessary to understand and predict these forces. The official investigation into the collapse recommended the use of wind-tunnel tests to aid in the design of the second Tacoma Narrows Bridge and resulted in the testing of all existing and future bridges across the country. New mathematical theories of vibration, aerodynamics, wave phenomena, and harmonics as they apply to bridge design arose from these studies.
SOURCE:
http://www.nwrain.net/~newtsuit/recoveries/narrows/narrows.htm

it was my belief that resonance was the cause of the collapse? the wind innitiated the resonance, which generated the upwards/downwards vibration. then, the integrity gave way, and it collapsed. am i wrong?

Yes, I think that is a fair assessment. I believe that the aerodynamic shape of the roadway was such that the wind would cause an upward force on it when it was on the way up and a downward force when it was on the way down. Sort of like timing your jumps on a diving or spring board, If your weight lands on the board at the bottom of its travel it increases that travel, and if it has the same frequency, or a harmonic frequency the amount of movement will keep on getting greater. In the end, if that can continue, the result is the destruction of the board or bridge. I don't know if the bridge guys would agree, but that's my opinion.