Assuming that the source generates pitch at a constant frequency, the source is likely to be continuously moving away from Jamie the observer.
Doppler Shift describes the observation that relative movements between the source and receiver would alter the apparent wavelength of both acoustic and electromagnetic radiation.
Waves from a source moving towards an observer are compressed. A stationary observer would find the wavelength shorter and hence the frequency higher than they actually are at the source. Similarly, waves coming from a retreating source are stretched and experience an apparent decrease in frequency.
A low-pitch corresponds to a low vibration frequency and hence long wavelength. The observation that wavelength appears to be increasing corresponds to the scenario that sound waves are being stretched when there's an increase in the separation between the source and the observer.