This happens if you put something on the passenger's seat which is not the weight of an adult person.
For instance, when my kid sits on the passenger's seat (weights 40 pounds), it deactivates itself automatically for security purposes (an airbag hitting a child in a car crash would certainly kill him).
This is very well explained in the user's manual (page 54 of my manual, Occupant Classification System (OCS))
It will do the same for a purse, backpack, some groceries, stuff like that.
It you remove what you have on the seat, the light should turn off and the air bag will re-enable itself within a few seconds.
Hope this helps.