Air Bag Misfired At 70mph For No Reason?

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I own a 03 plate Citroen C3.

 

Whilst driving at 70mph on the M1 motorway the driver side side impact air bag inflated for no apparent reason. (i know i hit nothing, & no marks on the paintwork) It was terrifying, i thought a tyre had burst. it hit my head and shoulder knocking my hands on the wheel making the car immediately swerve into the next lane.

 

Has anyone ever heard of this happening before - searching google i can find very few cases! i need answers. If the motorway had been busier or i was on my own i could defiantly have lost control, hit a car or rolled over and been killed.