The physical origin of resonant Wood's anomalies in the reflection spectra of three-dimensional (3D) opaline photonic crystals covered with Ge2Sb2Te5 (GST225) is discussed. For this purpose, the optical reflection spectra are studied for different incident angles of light both experimentally and theoretically. The performed eigenmode analysis reveals that the Wood's anomalies originate from the quasiguided modes which appear in the GST225 capping layer. This conclusion is supported by the simulated electromagnetic near-field distributions of incident light at resonant frequencies. The experimental reflection spectra are in good agreement with theoretical calculations performed by the Fourier modal method in the scattering matrix form.