Let’s take a look at two simple standing longitudinal waves (indicated by the spheres – note that the spheres are only visual aids).
If the two pulses have the same phase, the resulting field looks just like what you would get with two equal charges – a repulsive net force. This is is pure speculation, but I wonder if this is just coincidence or not…
The red lines with the yellow arrows show the resulting displacment of selecte points in the elastic solid.
If the two pulses have opposite phases, it looks just like opposite chrges (plus/minus) with an attractive net force: