What would cause my back lights to go out on my bmw x5.

It probably has corrosion built up....that happened to my X5.

X5 are notorious for having this issue. Almost 100% of the time the heat output from the taillights will cause the taillight circuit to melt. This further leads to high resistance and burnt contacts between bulb holders and the taillight circuitry. Remove the bulb holders and check if contacts are burnt. If so, the taillights will need to be replace. Hope this helps.

Also you might have to replace not only the bulbs but the whole circuit that you plug the bulbs into...

Taillights assembly are sold complete with bulbs and sockets.

I was driving in the night and the lights went out.