Its called tidal multipath. you are getting a reflected signal that is causing cancellation or attenuation from the AP (source). take for example a sign wave and mix it with another sign wave at the same frequency but change the phase a little or a lot you will get addition or subtraction, depending on the phase. when the waves are 180 degrees out of phase they will cancel each other, but as you shift phase they will attenuate or add to each other. when both waves are in phase with each other you will have a stronger signal. I am not a very good typist or writer, so I hope this makes sense. I have in the past plotted (graphed) RSL vs tidal height for microwave links over water. Its pretty neat to see the correlation of tide versus RSL.
Another great example of multipath was when TV was analog you would get ghosting. This is a great visual of multipath. As you receive a direct signal at a given point in time you are also receiving the same signal at a slightly delayed point in time. Thus a ghost image would appear due to the time delay. You also used to be able see the picture flutter due to Doppler shift mixed with multipath as an airplane would fly over at the right path.
Hope this helps in understanding on what going on.