I have seen several times Nova Scotia and/or Quebec joins the American Revolution. But whenever Newfoundland is mentioned in discussion it is simply said that they were too loyalist. So what nonASB would be needed for Newfoundland to join the Americans this could be with or without the rest of what would become Canada
Without the rest of Canada? No way. With the rest of Canada? Well...
The Albany Plan of union goes forward with lukewarm support from the British crown and strong support from more local colonial assemblies, possibly after a lucky French assault deep into British territory which greatly interrupts trade. With the signing of the plan and stronger power invested into a colonial grand council, the seeds are planted for a a more 'black and white' revolution. This grand council and unified colonial mindset manages to present a more unified voice of opposition from taxes.
Finally, the crown decides to disband the grand council and institute a wave of strong taxes.
This leads to less loyalist unity and strength, far fewer people taking a neutral stance on the revolution and stronger revolutionary fervor. More Canadian regions come along for the fight and the new USA wins more battles with a drastic change in the balance of power.