What were Patriots doing that was considered treason against Britain?

They were traitors, and at the time treason was punishable by the death penalty.

Explanation:

The term "Patriot" (ie loving and proud of their country) shouldn't really be applied to the rebels, until after the Declaration of Independence ... the year after the conflict began! Prior to that, the term would more accurately apply to, (of all people) the great many Loyalist colonists that took up arms Against the rebels.

When you rebel against a lawful government, you are considered a traitor. So, it's simple, to those who supported the rebellion, they were patriots. To those they rebelled against, they were traitors. To the British, those who remained loyal to the Crown were considered patriots.