It had been a trolley bridge in 1908, curving over the Deerfield River, uniting one section of town with the other on the Shelburne Falls & Colrain Street Railway. This company went out of business in 1928. The following year, the abandoned trolley bridge was turned into this Bridge of Flowers by some ingenuity and perhaps a desire to not see such a practical piece of infrastructure go to waste.
Volunteers, and donations from the public, and a paid gardener, keep the bridge this way, season after season. For more on the Shelburne Falls Bridge of Flowers, have at look at this website, and this one.