Because they didn't explicitly mention transwomen in their Women's Day tweets.
Lady Gaga said "We are the womb" which infuriated TRAs because of all the transwomen who don't have wombs. Lots of women have had hysterectomies and don't have wombs either but I don't see them throwing screaming tantrums on social media.
Rihanna's tweet involved an illustration of a woman pressing the button on a blender full of dicks. And of course this is transphobic because of the bounty of so-called 'female dick' out there...