Aroelastic fluttering.
Today marks the 70th anniversary of the collapse of the Tacoma Narrows bridge. Resonance was ruled out as the cause but it's an attractive explanation for us layman.
The oscillation and the energy within the bridge increased with time but the velocity of the wind was relatively constant. A 40mph wind when first applied can't by itself cause the deflection seen in the road deck.
Would someone care to decipher the technical explanation in the Wiki article?