Question: What theories might propose a casual reason for the Big Bang?
Discussion:
Evidence is strong that the mass in the modern universe was born approximately 14 billion years ago in a hyper-massive explosion that we call the Big Bang. But little is speculated about what caused the Big Bang in the the first place.
In order for the Big Bang to be a casual event (an event resulting from, caused from other prior events), then some form of universe must have existed prior to the Big Bang that lead to the Big Bang. Relativity theory teaches us that mass and energy are interchangeable. We can perhaps speculate that mass or energy or both existed in some form before the Big Bang explosion, and that that mass and or energy collected for eons prior to ending in a Big Bang explosion.
If we assume that the vacuum (or ether) of the pre-Big Bang universe was filled with massive amounts of energy, then perhaps we can imagine that this energy coalesced prior to the big bang. We know that when enough mass exists in a small enough space, it results in a black hole. But we do not know much about non-mass energy (vacuum energy), if it can be condensed, unstable or if it has the potential to explode into mass, similar to how a super saturated fluid can explosively crystallize. Physicist Ethan Siegel suggests that concentrated vacuum energy exponentially crystallizing into mass could explain cosmic inflation [1]. Could vacuum energy concentrations rotate around themselves before exploding/crystalizing. Or could the Big Bang have been triggered by a collision to two nearly equally sized hyper-massive black holes formed far away from each other that eventually collide and cause the universe to rotate. (It is unknown if the universe itself rotates, but could explain why we measure the universe as having an "axis of evil" and expansion increasing in speed farther from the center of the universe).
Conclusion: It seem likely that energy (likely vacuum energy, possibly condensed near the source of the big bang) existed prior to the Big Bang explosion, and that energy eventually exploded into what we know as the Big Bang. This conjecture is speculative, but consistent with big bang inflation theory.
[1] Ethan Siegel, August 3, 2018, There was no Big Bang Singularity "filled with energy inherent to space itself, which causes a rapid, exponential expansion", https://medium.com/starts-with-a-bang/there-was-no-big-bang-singularity-ae438985fcca