I"ve always found extra wide backing to shrink a lot. The store where I buy most of it has a big sign up to expect extra shrinkage and to prewash due to this.
I do prewash my backing and then have DH help me fold it up and put it away until I need it. When I need it wider, I run it length wise and cut off what I need, then put a horizontal seam to add the extra - ie if I need 108" I would cut one length at 118". And if it measures 100" wide, I'd cut strip at about 20" and then subcut it and sew it into one length, and add it to the bottom. I don't like long vertical seams for mounting to the LA, but shorter ones on the end don't cause the same problem.
I routinely make quilts wider then the backing so do this quite often.