2004 audi 2.0 fsi sport gearbox issue

[dinosoff]

hi all, i was driving my audi home along a bypass at around 75 mph at night. all of a sudden the car jammed on as if id knocked it into 1st gear by accident????? although the car was in drive! how could this possibly happen? nearly caused a major pile up....i have noticed that i can push the shift lever over from drive to 123456 maual without pressing button on side of shifter...any ideas please as i want to sell the car but dont know if theres an issue with it...thanks all