All known quasi-two-dimensional ferromagnets, such as (X=Br,I) or Cr2(Ge,Si)2Te6, exhibit a peculiar temperature dependence of the magnetization under small magnetic fields applied in the hard plane. Investigating the van der Waals layered Cr2Ge2Te6 by magnetization and specific-heat measurements under magnetic fields, we report the temperature dependence of the effective magnetic anisotropy as a plausible explanation for this unusual behavior. Magnetic temperature-field phase diagrams were measured for magnetic fields applied along the magnetic easy axis and in the hard plane up to 30 kOe and 150 K to obtain a detailed understanding of the magnetization behavior in the low-field region and especially below the magnetic saturation fields. From these magnetic phase diagrams the temperature dependence of the effective magnetocrystaline anisotropy constant is extracted and compared to the corresponding theory, unique for Cr2Ge2Te6 until now. Based on the thermal evolution of the effective magnetocrystalline anisotropy constant and the aforementioned magnetic phase diagrams, a qualitative scheme is developed explaining the changes of magnetization direction due to the influence of temperature, as well as strength and direction of external magnetic fields in Cr2Ge2Te6. Structural and magnetic similarities to other quasi-two-dimensional ferromagnets may allow this scheme to be generalized for the whole class of materials.