I say this as the mother of a vegetarian, though I do eat meat on occasion.
She is probably worried you are a bit of an extremist with the vegetarianism, and will influence your BF negatively in that direction. A lot of vegetarians make this lifestyle choice while looking down upon others for not doing the same... I'm not saying this is YOUR style, but it DOES happen quite a bit, and she is probably worried she will be judged by you. Also, she is not of a generation where vegetarianism is common, and she simply isn't used to it. People often react poorly to things they aren't used to.
I suggest you take the first opportunity to tell her you simply don't LIKE meat, and don't care what other people do, and keep any moralistic opinions you have to yourself, if what you want is peace. You are looking to fit in to HER family, and not the reverse, so this is the best way to avoid making waves.