What can cause a band to "roll up"?

Hello all,

I bought my mom a bra after measuring her using the calculator. (UK 40HH, US 40L) She told me it fits well and supports her much better than her old bras, but the band is rolling up a little in the back. When it's straightened out, she says it's very comfortable and feels like the right size. She does happen to have a lot of back fat, so I'm wondering if this is a common thing to happen? Any help is appreciated. I got her the Elomi Cate in white.