Mark... you're probably right. The extra agitation caused by edge sloshing seems like the likely culprit.
Michael... thank you. Yes, I've considered this method and may still do this. If I shoot two 5x7's side-by-side allowing for 1.25" overlap (more overlap makes the process easier) the final stitched image will be 12.5" wide (2:5 ratio). The Canham wooded 5x7 has more than enough rear shift to do this.
Keith... I'll seek out that article on Eckel's work.