First time I saw it I thought 'flattening elevations' because it was a topo (way long years ago) but then noticed it with large images also. Images, topo/grading plans (ie: elevation data) seem to be the worst, but large areas of hatch can cause it. Funny thing, some pdf's refuse to plot (adobe claims to have sent them but they never actually reach the plotter queue) with 'as image' selected, some refuse to plot with out it.