Same thing happened with me receently. Canson Platine 17x22 with a poster-image with some large areas of pure black under white lettering. Epson 3880. There was so much ink being laid down that the paper ballooned up in the middle from the dampness (I think), causing head strikes down the very center of the page starting about 6 inches in from the starting end. At least there was a line of strikes, transverse slash-like marks parallel to the direction the head moves, down the dead centre of the page, so I think it was due to the paper buckling up in the centre.