Thank you very much for your question.
There are a number of reasons why the air bag warning light may have activated and the fault code should be stored in the ECU (electronic control unit). I would recommend having a diagnostic check carried out which will reveal the fault code and point you towards the cause of the problem. You do not have to go to a main dealer for this check.
It may be something as simple as a loose connection, or more likely it could be a faulty sensor, but without the diagnostic check it's not easy to trace the problem. Remember though, air bags can be dangerous, so please take care if you check the connections, and in the event of an accident they might not deploy if the warning light is on or flashing.
Like the engine computer and ABS computer, the air bag control module runs a self-check every time the vehicle is driven. If it finds a fault in a crash sensor, one of the air bag modules, or the wiring then it will turn on the warning light.
I hope this answers your question.