Did you notice them blow? Or were they already blown for a while before you noticed?
Time and again I see this. Yes, it Could be the bulb, but as in the case of most low mounted fogs, be careful not to SHAKE them dead. Like not going over Railroad tracks at 40 or avoiding potholes. My Dad doesn't get it why his bulbs have gone again...he doesn't slow for bad tracks.
If you see how much movement there is in the bumper and imagine now going fast over bumps, it's quite easy for them to blow.