David Cay Johnston, the reporter under whose door the tax pages were slipped, noted that it was possible Trump leaked the documents himself.
Which, frankly, seems pretty likely.
I noticed (as did many others) during the broadcast that the tax pages bore a stamp reading "Client Copy." Further to that, I assumed Trump sent the pages when the schedules and attachments detailing income sources were absent, which is the information that actually matters in terms of establishing any improprieties, e.g.
Without that supporting documentation, these two pages are of little direct value to anyone but Trump, as they disprove the accusation that he doesn't pay taxes. (They only disprove it for a single year, but that's enough for his supporters.)
At the Washington Post, Derek Hawkins additionally notes: "Some even thought MSNBC host Rachel Maddow's apparent scoop on Trump's leaked 2005 tax return made him look good. After all, the New York Times had once suggested that he had avoided taxes, and others that he was faking the extent of his wealth."
The most important item to come from the docs, which unfortunately will probably get very little attention, is how his taxes were impacted by the alternative minimum tax. At the Guardian, Sabrina Siddiqui, Jon Swaine, and Julia Carrie Wong explain:
[T]he documents also showed that about 82% of the total paid to the Internal Revenue Service that year by Trump and his wife, Melania, was incurred due to a tax that Trump has said should be abolished.That Trump is a greedy and unethical opportunist with conflicts of interest who is using the presidency to enrich himself isn't, however, breaking news. This is another data point in an established narrative.
The "alternative minimum tax" (AMT), which was introduced to ensure the mega wealthy pay a fairer share of tax, comprised $31m of Trump's tax bill compared with $5.3m in regular federal income tax. In the run-up to November's election, Trump pledged to eliminate the AMT altogether, meaning the president campaigned for a change in the tax law that would have benefited him.
That doesn't make it unimportant, but it wasn't exactly a blockbuster, either. And I'm frankly not remotely certain that it was important enough to dedicate a (very overhyped) segment to detached tax pages that ultimately serve to help Trump.