One way of thinking about the distinction between E & B modes is *most* processes produce roughly equal amounts of E and B modes. After all, E and B can be thought of as reprojections of the Stokes vectors Q and U, though they have the major advantage being that they are independent of coordinate system. This is true of synchrotron emission, thermal dust emission, atmospheric scattering, instrumental effects (beam imperfections, instrumental scattering etc.) and even tensor perturbations due to gravitational waves. Unless I'm mistaken it is actually the scalar density perturbations which are special, and which don't produce B-modes. I can't remember the specific details without referring to Wayne Hu's website (or, perchance, my thesis, which was on B-mode excitement not too dissimilar from BICEP 1&2 in many respects!), but that is to do with the way in which they are generated by scalar fluctuations, and the resulting correlation between the direction and intensity of the polarised light.