Donald Trump married Melania Knaus in 2005. She had been a permanent resident since 2001. She became a US citizen in 2006. Part of the US naturalization checklist the last 5 years tax returns OR the last 3 years tax returns if married to a US citizen, which would be in Melania's case:
2003 single filing...
2004 single filing...
2005 joint filing with DJT
Melania's citizenship depended on a squeaky clean join tax filing in 2005. This is why Trump, yesterday, leaked his own (partial) 2005 1040 form. It was the only year he was forced to play by the rules.
This was a clear attempt to game the US public and media, distract from all the other shit that's going on, and make himself look as awesome as possible to his base.
If this was a 'leak' by a 'concerned 3rd party' there's no way it would be so beneficial to Trump.