The Great Awakenings in New England. Edwin Gaustad. 1957

Gaustad notes that 1) The Great awakening was a widespread phenomenon that crossed socio-economic boundaries, colonial boundaries, and denominational boundaries. It was "Great" in the sense that it was general, but also 2) that it was a struggle between the Enlightenment rationalism of figures like Charles Chauncey and the pietism of Jonathan Edwards.
Gaustad actually offers little explanation by way of causation. He only seems to hint at the fact that a perceived decline in piety may have impacted the religious inclinations of the time. He also notes the important role of itinerant preachers in bringing about the revival itself.