Maple sap is boiled to make syrup; to make sugar, further boiling is required before the syrup is ready to crystallize on cooling. Although sugar off and sugaring-off may have originally referred only to the second stage of this process (see 1852 quote), they have been commonly used with reference to the whole activity of boiling down maple sap.
1825  (1916)  Under his teaching Ailie quickly learned to sugar off, and did it over the kitchen fire in the biggest pot.
1836  The kettles should also, while warm, receive a thin coat of whitewash, and when dry, the sap, put in and kept boiling till it is "sugared off."
1852  (1923)  While Jenny was engaged in boiling and gathering the sap in the bush, I sugared off the syrup in the house, an operation watched by the children with intense interest.
1905  Sitting on logs, out in the woods beside the boiling kettles, the watchers had often a weary wait into the night for the time to "sugar off."
1957  Many thousands of maple trees will be tapped, filling sap pails which, when collected by sleigh, are taken to the evaporating pans and sugared off, producing the golden brown maple syrup, unrivalled for its taste and aroma, and blocks of maple sugar.
1965  He caught gaiety everywhere--coasting parties on the frozen rivers, canoemen shooting the rapids, families sugaring-off in the woodlot. . . .