These Belgian citizens were workers that drifted into unemployment following the crush of Belgian industry after the German invaded the country in 1914. By 1916, as German manpower began to decrease because of the high war casualties, they needed workers. So they basically enslaved those at this point 500.000 unemployed Belgians to work for their war machinery. Some of them were transferred to Germany, others had to dig trenches at the Western front and so on.
Source according to the link:
Records of the Great War, Vol. IV, ed. Charles F. Horne, National Alumni 1923