Ok, I am going to give you a not so long answer, but feel free to ask further questions if you want.

Short answer: it’s complicated. Something happened of which nobody is really sure why or how, thus giving wings to crazy theories and such… Which may be what the perpetrators intended originally. Let’s elaborate.

First, let's look at the question itself. The 23F, as it is known in Spain, was a very confusing event, but one that was readily understood as a very significant step towards "national unity" which is not a correct term in this sentence, but it is in this context; more than national unity, it meant the consolidation of the new parliamentary regime and the highlight in popular-legitimacy-creating act the king had to play. So it being so significant and confusing/secretive at the same time has meant a lot of questions have been asked and weird answerers proposed. It's such a popular topic in Spain that some years ago a very successful tv comedian-turned-socially-conscious-reporter created a very talked about fake documentary about the whole thing having been fake in the first place and shoot by a famous Spanish director, who confessed on prime time TV along with a cohort of very significant people of that time. The fact that a significant percentage of Spanish people didn't initially know what to think about its veracity points to the general darkness in which the event is still surrounded. That was, by the way, the declared intention of the documentary: there's so much important info to be officially told and accepted about what happened that night that even lunatic ideas, like the whole thing being a fake cinematographic creation (what you would call a "conspiracy theory” classic) are at least plausible if told by the right people the right way. 

The thing is: what actually happened (by that I mean, the actions taken by certain actors and its motivations) is more or less known in broad terms, and I'll tell you what it wasn't: it wasn't a "conspiracy" by king and government to bolster national unity. It was a very real political "conspiracy"/move by a lot of very important actors of the state, including the main political parties, parts of the army, the security forces and the intelligence agencies, various social and a economic players to actually depose the then current government in a time of heavy crisis in various fronts, and replace it with a National Unity temporal goverment, headed by a military man and formed by representatives of every main political party as potential ministers, including Ramón Tamames, a member of the PCE (The Spanish Communist Party, which had been the only anti fascist active organization during the franco decades). See how I took the crown out of that list, because that's pretty much were the bulk of the remaining "mystery" lies: its (degree of) implication.

At this point I need to clarify a very important fact, obvious for anyone aware of recent Spanish history but probably unknown otherwise: Since the restoration of the Monarchy in Spain, there was a tacit pact between the powers of the country about the figure of the King: as he was such a significant tool for Spain to not fall into civil war again, he needed to be protected, so there has always been darkness surrounding a lot of aspects of the life of Juan Carlos I: Spanish society had to rely on international press to learn about the corruptions, love affairs and dangerous friendships of the Head of State. This situation has changed in the last decade and now it’s more common to read investigative reporting about dark areas of Juan Carlos’s kingdom. I’m only saying this as context: up until the first decade of the XXI Century the King was untochable, so everything he did has always been surrounded by suspicion. The 23F event, being the one that retrospectively made him morph from the heir of the fascist dictator to the savior of democracy is, undertandibly, one of the most confusing points of his reign, and one of the most secretive. Thankfully times have changed, the political climate has changed and books that stayed in drawers for more than 20 years are being published. Even some protagonist have confessed and talked about it, in different books or publications. What we still lack is an “official” narrative that fills the gaps of the current one, as the current one makes very very little sense.

So, as Javier Cercas (an many others) in “Anatomía de un Instante” states, that night of the 23rd there happened not one, but two coups:. the one that failed by not happening, which is the misterious one, and the one that failed by happening as it happened, which (in a extremely ironic turn of events) ended up benefitting not only the interests of the instigators of the former, but the main goal of said failed coup, which was to get rid of the government.

Why would the whole political sphere of the country want to get rid of the President? That is a complicated topic, and has a lot to do with the Spanish Transition being more about the old Francoists keeping its privilegies in the new regime than about true regime change. Adolfo Suárez is a very interesting figure in Spanish modern history. It is agreed that he wasn’t very cultured or smart, but he was extremely charming and had a huge political instinct: after all, he went from being the head of the Fascist Spanish party to Spain’s first democratically elected president; he was the main tool the King and his entourage had to steer from absolute power to representative democracy. Suarez had become the enemy number one of conservative Spain when, after wining the first democratic elections, started to act by himself. One of the most important decisions he took, legalizing the Communist Party ahead of said elections had alienated big parts of Spanish fascist stablishment, and he became a traitor. Not only that, but things were hard overall in the early 80s in Spain: there was a terrible economic crisis and political violence was rampant, with both ETA having a particularly cruel spell and right wing terrorism reacting. Plus it was the time when the Estado de las Autonomías (the decentralization of power and recognition of “historical nationalities” in Spain, something that is still a hot topic), which was viewed by a big part of the Francoists as “the destruction of the motherland”, which was Franco’s ultimate fear: he had fought a war to preserve the “unity of the motherland” against traitor catalans and basques, and now franco’s successor and the bright kid of Franco’s party are about to give them independence. If you read the papers months before the coup, it is clear that something needed to happen. Or that was the strong idea shared by most of the political elites of the time. There was actually a very Spanish expression being thrown a lot those days in press: you could hear the horse of Pavía approaching. Pavía was a XIX Spanish General that ended the so called Sexenio Democrático (a 6 year period of democracy in Spain after the 1868 revolution) by literally riding his horse to Parliament and saying “enough”; In late 1980 and early 1981, the question was not if, it was when.