Generally, Blahaj and Hexbear disagree on what constitutes a chaser and thus Hexbear felt that Blahaj was not defending against chasers as well as they should, Blahaj moderators and admins were hostile towards Hexbear users, moderators, and admins, and general friction between the two instances. Additionally, some Blahaj moderators were ableist, using derogetory terminology such as "braindead" to refer to Hexbear users in removing their comments, as well as removing Hexbear users calling out ableism.
Are you sure about braindeath being a mental condition? My thoughts, confirmed by quick googling, was a medical-legal standard for death based on permanent death of the brain. As a person can otherwise be vegetative/comatosed or whatever, or like a liver can keep operating for a while etc.
As an insult I think it's meant to be taken as "This is only explainable if you have no brain activity" not "you are like a living human with a different or damaged brain".