I've written about the background of this here and in more detail below that here. Politico has a fairly good article on what happened after that which I'll add some analysis and historical context to.

Basically both Nixon and LBJ massively abused a tax loophole that allowed them to take writeoffs for donating their personal papers to nonprofits (mostly their libraries), stretching the definition of precisely what constituted 'personal' beyond any reasonable belief. And why not? The IRS would never dare to audit their boss (especially those two!), and there was now bipartisan precedent for using it. LBJ's abuse was probably more or less legal if morally decrepit and an incredible waste of government resources. Nixon's was trickier as Congress had mostly closed the loophole in 1969, so the real issue was whether or not Nixon had made the deadline and if he claimed parts of the deduction in later years how he accounted for doing so. However, since the document donations were only raised in litigation surrounding Watergate, Nixon was under no obligation to produce his returns unless compelled by a judge, and given how slowly those cases moved and how massively litigated they were, it was on the back burner. The Washington Post began sniffing around throughout 1973 in the context of Watergate and uncovered the fact the document donations had been made by both men, but there wasn't a number attached and it was still relatively unimportant.

Then a reporter for the Providence Journal made it extremely so. An IRS agent - who committed a felony providing the returns, although he was never prosecuted for it - provided him with a copy of Nixon's 1970 and 1971 returns. The headline material was very simple and damning: in 1970 Nixon had paid only $792.81 in taxes, and in 1971 he'd managed to pay only $878.03 while earning the rather substantial presidential salary of $200,000 per year (which had been doubled at the beginning of his term.)

In the midst of Watergate, this created a firestorm that Nixon couldn't afford, and Nixon agreed that he couldn't leave it unchecked and released his 1969-1972 returns at the end of 1973.

Except...what Nixon released to the public was fairly barebones in the form of the 1040s for each year. All the supporting documentation that you'd normally look through in an audit went to the staffers that assist Ways and Means and other committees on interpreting tax law, the Joint Committee on Taxation. In turn, they conducted a 4 month audit and in early 1974 wrote a 1,000 page report stating that Nixon should have paid north of $475,000 more in taxes (along with the IRS, who semi-freed from retaliation had conducted their own simultaneous audit, providing the slightly lower figure of taxes due of around $430,000) - or almost half of Nixon's reported net worth.

Given the way the laws at the time were written, there were also all sorts of legal landmines around what various members of Congress could ask for and actually state they found, so it became commonplace after that for all presidents and most candidates to simply release several years of the two pages of their 1040s to avoid the whole maelstrom that surrounded Nixon in those years.

As most presidents thereafter were comfortable but not wildly wealthy prior to taking office - the lecture circuit and writing memoirs after office is generally what moves them into the latter category - until now this hasn't been particularly controversial either in what was released.

Incidentally, there's no evidence that Nixon ever paid the additional tax bill, and I'd consider it probable that he never did so.