Not dangerous from how you've described it. I wonder if the car behind you slowed down as you set off (in other words, they were a nice, normal driver) and the woman behind was driving too closely to said car (or was on the mobile/applying make up/distracted by kids in the back etc etc) and had to brake hard due to not paying attention. Maybe from her point of view, you made the car slow down.
I wouldn't worry about it. I personally would've engaged her in conversation to try any find out what her problem was, and maybe given her a bit of a lecture on the importance of being patient and considerate to fellow road users and not blowing her top like a mad woman......and if she started shouting, just walk away making the 'Psycho' screeching violins sound.
