What causes this? 

Bad ground? Bad headlight switch? Bad relay?

If one is flickering probably a bad connector if both are most likely any of the above as @Mopar1973Man says.

Headlight and fog lamp grounds are in the driver side kick panel.

 

Fog lamp relay is in the PDC. 

 

There is another rare one if it just one bulb it could be the filament is broken and dance back and forth making contact and braking contact. My old aircraft landing lights were known for that one. Hit a bump the light would come back. Hit another and light would go out. 

Edited September 6, 20186 yr by Mopar1973Man

Checked the grounds and thy are clean... 

Even though the drivers side kick panel ground looks clean I sanded mine anyway and removed the factory chrome coating and my headlights got brighter from just doing that.

1 minute ago, JAG1 said:

Even though the drivers side kick panel ground looks clean I sanded mine anyway and removed the factory chrome coating and my headlights got brighter from just doing that.

I’ll try that!

3 minutes ago, Marcus2000monster said:

I’ll try that!

Usually a flickering is a bad connection where a wire will be shaking all the time. The constant shaking of any wire can cause it to eventually break its connection. I would have the fogs on and start working from underneath moving the wires around a bit till you find it. keep moving around to other wiring involved till you can make it flicker yourself. I've even found bulb sockets to be corroded and had to clean them. I use dialectric grease of any bulb socket dealing with the weather outside.