The Post's Aaron Blake has three possibilities, which are not mutually exclusive:
1. There is something nefarious going on
If there is something nefarious going on, a private, undisclosed conversation that was reportedly out of earshot of other world leaders would be a great place to do it. And given the Russian government's and Trump's track records, it's not like we're going to get a straight answer on what they talked about.
2. Trump is oblivious to how this might be perceived
I've framed many of Trump's actions under the rubric of Adam Carolla's “Stupid or Liar” theory before. This reason would be the “stupid” part of that equation.
3. Trump is simply addicted to causing controversy and/or sees it as a GOP base play
[A]t this point, Trump and his team have to be wondering: What's the payoff? What is he really getting out of it? Trump's approval rating is the lowest in modern presidential history, the GOP-controlled Congress hasn't passed any signature legislation, his party split on one of his major promises on the health-care bill, and all Trump has to show for it is a mostly intact group of Republican voters who say they still like him.
I'm betting on all three, though #2 may be the root cause.