Hexter first did this in an essay entitled "The Burden of Proof," a review of Christopher Hill's book Change and Continuity in Seventeenth Century England that appeared in the October 1975 edition of the Times Literary Supplement. The essay sparked a very famous exchange between the two historians that has become a classic in historiographical debate. Hexter later published "The Burden of Proof" in a volume of his essays entitled On Historians. The categories of "lumpers" and "splitters" had been around for some time before Hexter's essay, and describes what the historian perceived as two different types of thinkers: "lumpers" tended to note similarities and consistencies between things, and "splitters" tended to focus more on the differences.
Hill was a Marxist historian (though far less reductionist and deterministic than some others, especially by the 1970s) and Hexter argued that he allowed his predetermined conceptual framework, one which emphasized the transition from feudalism to capitalism in seventeenth-century England, to locate it in evidence where, Hexter thinks, it did not exist.
In particular, Hexter took issue with Hill's contention (following Max Weber) that seventeenth-century Puritanism was instrumental in, and in many ways a reflection of, the transition to capitalism. Hexter arued that Hill, unquestionably a "colossus of erudition," had "source-mined," selecting information from sources that supported his thesis, while ignoring information that did not. As a "lumper," Hill had to make history fit into neat boxes, and anything that does not fit into these boxes reflects not the "untidiness of the past but the untidiness of the records of the past."
Sources: William Palmer, "The Burden of Proof: J.H. Hexter and Christopher Hill," Journal of British Studies 19, vol. 1 (Autumn, 1979)
J.H. Hexter, On Historians: Reappraisals on some of the Masters of Modern History (Cambridge, Mass.: Harvard University Press, 1986)