What explains the colonists' shift from resistance to outright rebellion?

The shift from resistance to outright rebellion was a gradual one. There was no one single cause or catalyst that sparked the American colonists' rebellion against the British authorities. None of the numerous steps that marked the final breakdown in relations between America and the mother country was inevitable; none of them needed to happen, and all of them could so easily have been prevented.
But they weren't. And the fact that they weren't is largely due to a failure in communication between successive British governments and the American colonies. The Americans had come to see themselves as inheritors of the British tradition of liberty, of which the Glorious Revolution was the most impressive manifestation. The British, however, did not subscribe to the American colonists' elevated self-image. Over time, they came to regard the American colonies as a cash cow, a reliable source of income needed for the British state's expensive upkeep. To successive British governments, talk of "inalienable rights" and "No taxation without representation" was just so much cant. Though forming the creme de la creme of colonial society, the leading figures in American politics were considered by the aristocratic British rulers as little better than parvenus, whose counterparts in Britain were also denied representation in Parliament. If they could be taxed without representation, then why couldn't the Americans?
This supercilious attitude never ceased to antagonize the American colonists. And to make matters worse, the British authorities gradually introduced a series of measures which seemed almost deliberately designed to add fuel to the fire. The Stamp Act, The Townsend Duties, and The Tea Act all expressed the enormous and growing gulf in the British and American attitudes to the whole purpose and rationale of the American colonies.
Yet even with all the mounting anger, the open challenges to British rule, and the acts of civil disobedience, the majority of Americans still believed that some kind of amicable settlement was possible. As late as 1788, Patrick Henry had to employ his immense rhetorical abilities to the full in order to convince his fellow Virginians of the overriding necessity of rebellion. He needed to work hard; for most Americans, the very idea of rebellion was dangerous, a leap in the dark.
However, Henry and other advocates of a final breach with the British were ably assisted by the mother country herself. As Henry himself made clear, and as the Declaration of Independence spelled out in the starkest possible terms, the colonists had made repeated attempts to reach an amicable settlement, only to be condescended to, ignored, and rebuffed. Again, the radically different attitudes of the respective parties ensured that the only kind of dialogue possible was a dialogue of the deaf. The Americans and the British were speaking different languages, and so long as this was the case, no peaceful settlement between them was remotely possible. The growing realization of this uncomfortable fact among the American colonists, combined with the stubborn intransigence of the British, meant that, though rebellion was still far from inevitable, it gradually became the only serious means of settling matters once and for all.