So I think we know why Donald Trump suddenly decided to attack the Mueller investigation publicly.

Courtesy of the New York Times:  

The president’s tweets, posted on a Saturday in which he remained inside the White House with no public schedule, came as Mr. Mueller is said to have sent questions to Mr. Trump’s legal team as part of negotiations over an interview with the president. Mr. Mueller is seeking the interview, according to two people close to the White House, in order to ask follow-up questions, but put forward the list as a start. 

Mr. Dowd’s remarks about Mr. Mueller’s investigation represented an extraordinary shift in public strategy for the Trump legal team. Since taking over the case last summer, Mr. Trump’s lawyers have urged a strategy of restraint, in which the president avoids discussing Mr. Mueller or criticizing him, and the lawyers had done nothing publicly until now that could agitate the special counsel’s team. 

The comments by both Mr. Trump and Mr. Dowd lent credence to Mr. McCabe’s assertion that the president sees his firing as directly tied into Mr. Mueller’s case. Mr. McCabe, who is a potential witness in the investigation, declared that his dismissal was an attempt to undermine it.

Why does the Mueller team have 13 hardened Democrats, some big Crooked Hillary supporters, and Zero Republicans? Another Dem recently added...does anyone think this is fair? And yet, there is NO COLLUSION!

— Donald J. Trump (@realDonaldTrump) March 18, 2018

According to Axios these are the questions that Mueller sent to Trump's attorneys:

Axios has learned that special counsel Robert Mueller has focused on events since the election — not during the campaign — in his conversations with President Trump's lawyers. The top two topics that Mueller has expressed interest in so far: the firings of FBI director James Comey and national security adviser Michael Flynn. 

That suggests a focus on obstruction of justice while in office, rather than collusion with Russia during the campaign. But both sagas are interwoven with Russia: Trump himself has linked Comey's firing to Russia, and Flynn pleaded guilty to lying to the FBI about conversations with the Russian ambassador during the transition.

In other words Mueller may have Trump locked down both coming and going.

It has been clear for awhile that Trump's attorneys have barely managed to rein him in when it comes to tweeting out his frustrations with Mueller's investigations, but now having seen the questions that Mueller intends to ask him, and realizing that answering them under oath will be very bad for his presidency, I think that Trump is losing his shit entirely and only sees the firing of Robert Mueller as his only possible way out.

Of course if he actually does finagle a way to get rid of Mueller that will essentially e an admission of guilt, and then this whole things ramps way up.