how did the molasses flood start

The tank of molasses at the Purity Distilling Company burst at 12:30 in the afternoon of January 15, 1919.

See the link below for more on the tragedy.

Yes. The great molasses flood occurred in 1919, and it covered a large part of Boston with the thick, gooey substance. It took a long while to clean it all up, and for many years, people in the area claimed that on a warm day, they could still smell the molasses. These days, that claim is more of an urban legend than a fact. The disaster occurred nearly 100 years ago, and much has changed in the city of Boston, including new roads and buildings. It is doubtful that the smell of molasses really lingers today.

Boston is known as Beantown because Boston was part of the triangular trade and traded many goods with the west indes for molasses there for Boston had many molasses beans were baked in the syrup(molasses) and they became very popular in Boston therefor Boston became well-known for its beans giving it the name of "Beantown"

Use salt water, as was used in the Boston Molasses Tragedy of 1919. Warm salt water works best.