What was the catalyst for the Big Bang?

As every biological, chemical, and physical reaction or occurrence must have an original instigation for initiating ... 

It's fine to hold that something is most likely to the case, based on observation in its normal contexts, but the moment you assert a "must", as an absolute, your position is in trouble. When we're talking about the Big Bang, we're already discussing a scenario that is so bizarre and warped, that our current modern physics - and even our very concepts of time, which are woven into causality - break down. 

Keep in mind, that this "we always observe XYZ, so it must always be XYZ" cuts both ways. We also have always observed that natural things that begin to exist, were caused by other natural, material things. So therefore, the universe's cause was a natural, material thing. The only wiggle room is that it could be an intelligent, material, natural thing, since we've observed a mixture of intelligent/non-intelligent causes.

So asserting a "must" here, is an overstatement. Science frequently assaults our intuitions and "common sense".

The idea of cause-and-effect is up for scrutiny as well, particularly when we're not talking about observations on the normal, day-to-day operations of spacetime, but are are trying to apply causality to a point so extreme, that it seems like time itself "began". 

Something is off, like trying to apply gravitational equations close to zero-distance from a black hole singularity - it goes haywire. That doesn't mean the Law of Gravitation is totally wrong, or useless. It means that it's incomplete, and only applicable in the correct conditions.

what was the cause or catalyst that precipitated the explosion commonly known as the Big Bang?

Hell if I know... assuming there was one.

Whatever the answer is, the appropriate time to accept it, is when it's been supported by substantial amount of hypothesis testing, peer review, and falsification attempts. 

So, not right now.

Also, "explosion" may not be the right word. Physicists seem to prefer "inflation". If that was an explosion, then it's still "exploding" now... but now we're picking nits.

Also, how is this theory compatible with the scientific laws of the conservation of mass and energy, which state that both matter and energy can be neither created nor destroyed?

Because those laws may be wrong, or incomplete too?? Consider Newtonian mechanics - often used for projectile motion calculations. Within its original context of investigation, it still "works", and is valid. But we expanded physics into contexts where Newtonian mechanics break. That would be with general relativity or quantum physics.

Even those two are having a knife-fight in some contexts, so there's something wrong/incomplete with one/both of those. 

That's the march of science.  Better, more comprehensive theories replace and expand the previous.

Another idea is that the Big Bang didn't create/destroy any matter/energy. The energy/matter ball/singularity of whatever it was, existed as it was, in an unchanging state going back "forever", for lack of a better non-time way of putting it. The Big Bang event was more the point where the universe "began" as we know it.

But until the gears of science can figure out the answer (if it even can), the correct answer is "we don't know". Nothing more. Nothing less.

Anyway, back to atheist-related questions...