This is an excerpt from my book Florida and the American Revolution

In December 1775, Continental Army General George Washington was informed that the British had been stockpiling weaponry, ammunition and other materials in St Augustine. At this point Washington began to prepare defensive actions in the event that the British launched an invasion of the southern colonies from East Florida. Additional fortifications were ordered in Savannah and other outposts were reinforced. 

The concern was so great that within weeks the political leadership in Philadelphia decided they’d rather take offensive action against East Florida preemptively than wait for the seemingly inevitable British invasion. 

In January 1776, the Continental Congress authorized the southern colonies to attack St Augustine and capture it in order to safeguard the Patriot cause. 

The Congress resolved:

“That the seizing and securing the barracks and castle (sic) of St Augustine will greatly contribute to the safety of the colonies.”

Additionally, capturing St Augustine would give the Continental Army control of the Castillo de Marcos and the ability to directly fight any invading British Army coming from the West Indies as a result. The fort had been held as a Spanish outpost against British sieges in both 1704 and 1740.  

A serious miscalculation was made in the Continental Congress, similar to the mistakes made in the Quebec campaign. This was an assumption that the general population of East Florida would rise up if given the opportunity.  St Augustine by this time was filling up loyalists and runaway slaves and also had many Crown-aligned native Americans either in the town or nearby.  After the collapse of New Smyrna, the area around Saint Augustine represented the only major population center in East Florida.