After posting the above mini-theory, I kept thinking about what potential alternative explanations there could be for Trump to talk to Mueller "voluntarily". I came up with one alternative idea, but it's pretty wild and would only fit the most extreme circumstances behind the Trump Russia scandal... In other words, if you're interested... be sure to get your tinfoil ready, crank up the X-files theme music, and enjoy this thought experiment with many grains of salt:
So, first off, keep in mind that the Trump-Russia probe isn't just about Trump's actions alone - it's also a counter-intelligence investigation to find out exactly how Russia interfered with our election. Mueller and the other Congressional investigators need a full account of what happened - including how Russian power/influence/money was leveraged through US persons to do things like hacking, micro-targeting, voter suppression, and even the hacking of the voting machines themselves. It's a critical mission so that our country can protect our democracy from future interference.
Ok, with that in mind, let's consider the scenario in which Trump did colluded with Russia/Putin/GOP donors & politicians to interfere with the election. In addition, assume that the other allegations outlined in the Steele dossier are true as well, with Trump also guilty of (relatively) lesser crimes like money laundering, tax evasion, bribery, fraud, etc.
Now, put yourself in Trump's position, knowing that, in this scenario, you're guilty of all these accusations. Currently, you're realizing that Manafort and/or Flynn are likely to be indicted soon, and they're a risk to you since they could rat you out in return for lighter sentences, especially if they don't think you're going to be able to pardon them. However, based on your conversations with your other co-conspirators, you don't think Mueller's going to be able to prove that you're guilty of your worst crimes... but he's getting dangerously close and it'd be inevitable that he'd get to the bottom of it given enough time.
So, Donnie, what's your best option here? Sure, you could try to pull a Nixon and fire everyone you need to get Mueller fired, but that didn't turn out so well for Dick and would only make things worse - as you've learned your lesson from firing Comey. You could continue to deny the allegations, distract the public with other shenanigans, and try to hold onto your base to hold the Republican party hostage to maintain political power... but the truth is that your civil war with the establishment GOP is about to break wide open - Corker's leading the charge, and the party senses that Bannon/Mercer may be a stronger political force than you with the election of Moore over your endorsement of Strange. The reality is that the rest of the party, including your potential co-conspirators in the election interference, are ready to cut you loose in order to stick you with all the blame for the lack of legislative accomplishments so that they might hold onto their power through the 2018 midterms. In other words... your presidency is already over, as you can no chance of getting any major policy goals done.
Given all this... especially the prospect of Mueller closing in the dark truths... why not end it all ASAP? If you talk with Mueller now, and self-incriminate yourself on lesser charges, then you can control how quickly the investigation ends by going down now. The Republicans are just waiting for your indictment to openly turn on you, and once there's enough impeachment momentum you can resign the presidency and active your contingency plan - have your successor pardon you for those minimal "witch hunt" crimes and pick up right where you would have left off if you lost the election - railing against the deep state/swamp, perhaps on your own new media network geared towards your still devoted base. Putin would be happy that you're continuing to weaken the US by sowing discord and dividing the people, and would continue to reward you financially; and could even manuever for a repeal of the Magnitsky Act if Mueller couldn't get to the bottom of the conspiracy before your pardon.
So, if you talk now and take the hard fall, you'd hamper the investigation from proceeding any further, since the remaining party members would push for a quicker resolution to the investigation(s) to "let the country heal and move forward". Why continue the "witch hunt" after 'ding dong' the witch is dead? Ending the investigation before it can uncover everyone protects everyone one else involved, allowing them to iterate their tactics for the next election...
This would be a terrible end to your presidency, but given that the alternative would risk implicating those in position to provide leniency to you, you'd talk to Mueller now before he can bring the entire gang down. If you wait too long, then you'd lose the option of a soft return to your business empire, and instead would die disgraced, penniless, and incarcerated.
... so yea, that's my alternative grand master conspiracy theory as to why Trump would be willing to voluntarily talk to Mueller now. I think spinning a Mueller request to testify makes sense in many more scenarios that don't require bigger conspiratorial assumptions (and would still make sense even if they were all true).
Still, has the Trump presidency gotten more or less weird over time? and would you predict things to get more normal from here on out... or do you expect things to get weirder and weirder?