Actually, I could easily see how this could happen with a FF BLVD.....
I took off our black panel when I turned our ADV (basically a smaller version of the BLVD) forward because the splitter plate kept 'catching' on the long vertical bar that holds the adjustable headrest, and I could not tighten or loosen the straps unless I manually reached through the side of the carseat to 'un-hook' the splitter plate from the bar. I couldn't even get AT it with the black panel on - so I took it off.
My straps were constantly getting caught, and I would try to pull them free from the front, which means that the splitter plate was probably being 'jerked' upwards.
My seat was installed w/ a lap-only belt, but if there had been a shoulder-belt there - i guess I could see this happening!