The Multi-stAge noise SHaping Digital Delta-Sigma Modulator (MASH DDSM), which is commonly used as a divider controller in a fractional-N frequency synthesizer, can produce a time-varying periodic pattern of tones in the output spectrum of the synthesizer; these are known as wandering spurs. In this work, a detailed analysis of the cause of wandering spur patterns in a MASH 1-1-1 DDSM-based fractional-N frequency synthesizer, supported by experimental measurements, is presented.