Q: According to some reports, Trump plans to oust Defense Secretary James Mattis after the congressional election. What are the reasons for this?

A: There are many reports that Mattis has, in effect, not followed the orders of the president.  Whether this is the real reason, or whether there is any animosity between the two is unknowable. The apparent relationship between them is friendly, but by most indications Mattis will be gone by the second term. 

Q: Why would Trump postpone this decision until after congressional elections? And how important are these elections for him?

A: Mattis is very well regarded by both democrats and republicans, so to fire him would be an unpopular thing and would probably be harmful to republicans in the midterm elections.  So, they would want to wait until after it, if he is to be fired.  

The congressional elections are very important for Trump because if the democrats take over then they have the power of investigation and the power of subpoena, and so, as frequently happens in American politics the party out of power may have the House, and therefore the ability to make life miserable for the president.