When did the greatest drop in british imports happen and what caused it?

The largest drop was after the 1774 Intolerable Acts.

Explanation:

Intolerable Acts of 1774 were a set of minor laws passed by the British after the Boston Tea party, which was the beginning spark of the American revolution.

These laws were introduced to punish the Massachusetts colonists who stood up against the unfair and unbearable tax changes made by the British government.

In fact, there were 5 Intolerable Act. I've briefly summarized them below so you can google more and learn more!

1st Intolerable Acts - March 31, 1774: Boston Port Act 2nd Intolerable Acts - May 20, 1774: Massachusetts Government Act 3rd Intolerable Acts - May 20, 1774: Administration Justice Act 4th Intolerable Acts - June 2, 1774: Quartering Act of 1774 5th Intolerable Acts - June 22, 1774: Quebec Act 1774