I think it's due to 'curvy' nowadays referring to a particular kind of 'thick' or a smallfat who has a very favourable fat distribution(all hips, butt and thighs and they look like a somewhat evenly bloated curvy girl). Most fatties don't fit this mold because they sometimes have huge FUPAs, bingo wings, double chins and other unpredictable 'ugly' fat distributions. So #alternativecurves is sort of a movement to promote acceptance of fucked up body shapes(or the unacceptable fat body shapes) as normal, visible and beautiful.