Mass, absorbed at the surface, decreases velocity of the surface acoustic wave. SAW sensor launches the SAW with fixed wavelength, determined by the period of the interdigital transducer (IDT). Frequency is velocity / wavelength, so mass loading modifies the frequency.  

Sometimes the frequency can be increased, if the absorbed material is "faster" than the substrate.

Simple explanation for the velocity change could be that velocity depends on the stiffness/density ratio. If you add mass without increasing stiffness, this ratio decreases.

Mass loading to SAW will change its resonant frequency. If you know a frequency shift, a mass can be known as long as the SAW calibrated (Mass - frequency relationship) prior to measurement.