because he had to redo the list of foreign contacts on his request for a security clearance. The speculation was that it was better for knowledge of this to come out associated with Trump Jr, who has no role in the administration than it would be for Kushner, with the highest clearance there is, to have to speak about it before Congress.
I suspect that the NYT had some hint of this from their own investigation and was asking the right questions - making it obvious that this WAS going to come out - and there was an administration assesment that it would be less damaging if Trump Jr was seen to be transparent about it. Remember that at least a week before, many RW media voices switched from no collusion to "that would not be illegal if it happened". In hindsight, the Trump response when they saw the media was going to get and publish this story -- was everybody would do this, give Jr credit for transparency, and HRC was far worse. (Having been a parent, imagine your kid fearing a phone call from the school, telling you that Hillary and Billie were way worse than him and he didn't do anything in class that otheres wouldn't have done given the chance. )
There are some stories that Manaford or people with him might have been the source, but I don't see how this takes pressure off of him.