Quantitative information about tie growth-induced magnetic anisotropy in Y1G was obtained from M?ssbauer spectrum in a previous note. In order to study the origin of this anisotropy, M?ssbauer measurements were extended to low temperatures. We discover that below 170 k, Ku is negative, i-e., there is a spin reorientation. Calculations show that single-ion anisotropy of 0.01/Fe2+ may account for the observed Ku(T), if Fe2+ ions are assumed to occupy preferentially octahedral sites except (1/4 1/4 1/4) and (3/4 3/4 3/4).