I'm given to understand that the Jacobite Movement's primary motivation was at least nominally religious, hoping to restore James II and VII, a Catholic monarch, and his heirs to the English/Scottish and later British throne, as well as fighting against persecution of Catholics. Furthermore I have the impression that Scotland, a Protestant country, was the major stronghold of the Jacobite Movement. Were Scottish/English/British Catholics simply a large or prominent enough minority that they could launch major rebellions like this, or were there reasons other than religion that Protestants might join in the uprisings? If so, what would those reasons be?
Absolutely yes - the actual number of Catholics on mainland Britain was so small, that there's no way the Rising could have sustained itself for as long as it did if it had relied only on Catholic supporters.
Jacobitism had a degree of support from Protestant Dissenters (those who didn't adhere to the established Church of England), as James II had attempted to introduce general religious toleration during his short reign, motivated primarily by his desire to facilitate Catholic practice. Scottish Episcopalians were especially supportive in both the 1715 and 1745 uprisings.
There was also a degree of Jacobite sympathy among Protestant English Tories who, although not welcoming of Catholicism per se, felt strongly that James II's line was legitimate, and that his anointing/coronation could not simply be undone by William III's usurpation.
Finally, by the time of the 1745 rising, there was a small number of English Protestants who were simply dissatisfied enough with the existing Hanoverian/Whig hegemony to support the Jacobites, regarding the government as corrupt and the continual foreign wars as ruinously expensive. In northern towns like Manchester, some Protestant inhabitants supported the Jacobites as they marched south, simply as a protest.