A peculiarity of AxisLocation='origin' is that the X and Y tick labels automatically switch location (from below the X axis to above-it, and from left of the Y axis to right-of-it, respectively) depending on where the axes origin (0,0) is located. You can easily see this behavior by dragging (panning) the axes up/down and left/right, respectively.
If you want to ensure that the labels appear beneath the X axes, then make the YLimit above the axes larger than the one beneath it. For example:
ax.YLim = [-.80001,.8];
axis([-6 19 -0.80001 0.8]);