I knew that Hessian mercenaries fought in the Revolutionary War, but I didn't know that they fought in the Civil War.

There were probably as many reasons for "switching sides" as there were men who switched sides. Fear, revenge, playing the odds, etc., were surely all factors. However, we shouldn't overlook the fact that for most of the people of the Upper South, there was great reluctance to break ties with the old Republic. Lincoln's call for troops to suppress the Deep South states was what finally pushed them over the edge. Wartime letters and diaries, as well as postwar reminiscences, from quite a few Confederates reflected a fondness for the old Republic and the Stars and Stripes, and regret that secession was deemed to be necessary. With such sentiments, combined with other factors, it is not surprising that so many of them switched their allegiance back to the United States when the opportunity presented itself.