Excerpts from Books and Wikipedia
"They were protesting about the king's refusal to repeal the Catholic Relief Act, which removed most of the disqualifications placed on Britain's Catholics in 1689 under William III. In 1778, impelled by a severe shortage of troops to fight the U.S. War of Independence, the British government had passed the act to boost Catholic recruitment into the armed forces and it was given assent by King George III." [1001 Days]