I have inside sources so here goes:
Flight attendant found out life threatening allergy and called Ops agent.
Ops agent confirmed life threatening and asked for medical certificate.
She was offered first flight the next day by Ops agent and asked to deplane.
Since she didn't deplane, three cops approached her one by one and asked her to deplane. Then the captain asked her to deplane. Cops then forced her off.
Asked to deplane because it was life threatening. If just allergy, they'd create a buffer zone. Needs a medical certificate dated in the past 10 days saying ok to fly despite life threatening allergies. Due to no certificate, ESA trumps allergy. If she had a certificate, it would be the first one (between ESA and allergy) to check in stays.