McGahn has been in the headlines
There have been a couple big stories about McGahn lately. The first is that he was cooperating extensively with the Mueller probe and may have implicated Trump (NYT)
The initial report said it was because he feared he would be thrown under the bus by the White House if an obstruction of justice charge ever came, so he was covering himself.
The White House responded that he was cooperating extensively because they had nothing to hide.
That story seemed to blow over at the time but more recently he was in the news because Trump was asking for advice on pardoning Manafort, or asking him to prepare some documents for it and he refused to be a part of it (Vanity Fair). The story said he knew it could cost him his job and didn't care.
Axios reported on McGahn leaving earlier and says Emmet Flood may get the job, who was a Clinton administration lawyer.
What does it mean for markets? Probably nothing but we're getting close to midterms and soon trading is going to be even more political. I doubt Trump would pardon Manafort before the midterms but you never know.