I agree. I think Mrs. Dashwood is particularly affronted by Fanny's insinuations that Elinor is trying to 'draw him in'. The very notion of a young woman trying to use some sort of scheming or deceit, rather than love, esteem and affection, to get a young man to propose to her offends Mrs. Dashwoods romantic sensibilities in the extreme.
[Fanny] took the first opportunity of affronting her mother-in-law on the occasion, talking to her so expressively of her brother's great expectations, of Mrs. Ferrars's resolution that both her sons should marry well, and of the danger attending any young woman who attempted to draw him in ; that Mrs. Dashwood could neither pretend to be unconscious, nor endeavour to be calm. She gave her an answer which marked her contempt, and instantly left the room, resolving that, whatever might be the inconvenience or expense of so sudden a removal, her beloved Elinor should not be exposed another week to such insinuations.