I think atonement is the core theme about how both these men let their personal issues get in the way of the case being reopened in 2002, and how they have to set it right this time. For Marty, he owes Rust because he partly fucked it up in 1995 when he killed the suspects and Rust covered for him. For Rust, this is about not leaving an important job unfinished.
The question though, is why now? Change doesn't happen randomly when a person has settled into a pattern. It's something the show has taken care to show in the character arcs. Major change happens in their lives because of specific events which push things forward. The Dora Lang case brought them closer together as partners. Killing Reggie LeDoux helped save Marty's marriage for a period of time. Maggie sleeping with Rust tore them apart.
So in 2010 something must have happened to Rust. Something he's not sharing yet, but it pushed him to return to the state, break the law, find compelling evidence, and continue this crazy investigation he dropped 8 years before that. It's not going to be a "OMG WTF" plot twist either, because the story has always been about the characters and how they related to the world and others around them. So I think him having to confront his own mortality and wanting to make the best use of his time before he goes, fits with all the other themes too.