Why was the Muslim American woman thrown off the flight according to Southwest Airlines?

A Muslim mother-of-three was thrown off a flight at San Diego International Airport, California, because one of the crew thought her behavior was "suspicious".
    Irum Abassi, a graduate student in experimental psychology, believes she was discriminated against because she was wearing an Islamic headscarf.
    Southwest Airlines has made a public apology to the 31-year-old, who was told a flight attendant believed she said "it’s a go" - or words to that effect - on her cellphone.
    But Mrs Abassi told reporters at a news conference yesterday she actually said "I’ve got to go" because the plane was about to take off.
    Southwest Airlines apologised to Mrs Abassi on the day of the incident, gave her a voucher for another flight, and apologized again over the phone on Wednesday morning.
    But at the news conference, the student said the verbal apology "doesn’t make me feel better. This time they said we weren’t comfortable with the headscarf. Next time, they won’t be comfortable with my accent or they won’t be comfortable with my South Asian heritage".
                                                                    Dailymail