God As Grounding Cause

[robvalue]

Why can't all of reality have popped into existence for no reason at all? Or does "no reason" itself qualify as a cause?

Pretty sure you mean "no reason" (we know of).

If magic was a possible thing in our universe, then sure, a malfunctioning wand could poof a unicorn with 3 horns into existence for no real reason.
The only way God (as described by the bible) can exist is if magic can exist.

Which then begs the question, why do we then even need to use logic to justify God's reason's for doing anything at all?
Maybe God gives kids cancer because tomatoes are red

A totally batshit claim for those without indoctrination.

I was thinking of a scenario where it just happens. Reality starts to exist, and the only reason/cause is the fact that it's happening.

Of course, we have absolutely no reason to think that this was actually the case. But my point is that such a scenario cannot be excluded simply because it doesn't gel with how the theist imagines things working, just in order to leave us with a specific cause. (They also can't exclude everything having always existed in some form, with no initial point.)

If this scenario isn't being excluded, then "God" could be absolutely nothing at all. I've heard some people actually take this stance: a kind of compatabalist deism. I find that to be a pointless tautology, but whatever makes them happy I guess.