The charts suggest two features: (1) line C, using the data of a more recent observation period, appears to be shifting downward, almost parallel to line B; and (2) the slope of line C appears to be flatter than that of line A, for which the observation period is longer.
While various factors influence both features simultaneously, let me list some that may be considered to have affected each of these features.5 It is possible that some of the factors affected both the shift in the Phillips curve and its slope.