The question is loaded. It is assuming it is a "who" (some kind of being), instead of a natural mechanism.

While a bunch of theories exist that have yet to be supported, let's cut to the chase.

We don't know.

Just because we don't know does not mean there's any credibility to asserting that a supreme being is responsible. When we say that we don't know, it means we don't know - nothing more, nothing less. We're currently looking into it.

This counts for virtually every question of "Well, if there's no God, then how do you explain ________?"

Humanity has a long history of investigating phenomenon (such as lightning) that were thought to be supernatural (thrown by Zeus), only to find they are perfectly natural phenomenon (equalization of electrical charge between the upper atmosphere and the ground via electrostatic breakdown of the air).

Out of that huge list of scientific investigations, we as a species have never confirmed a supernatural claim.

This question can arise often from a misconception about the burden of proof - "Can you prove that God does not exist?".