Economic strength and popularity of the sitting president impact many candidates' decisions to run for congress, an idea referred to as:

"Strategic politician hypothesis"

Politicians will act in accordance with their own best interests.  If circumstances in the country favor incumbents, incumbents will be likely to run for reelection.  If the tide is seen swinging the other way, they may decide to "retire" or not run again.  In the present political context, a number of Republican members of Congress have announced that they are not running for reelection, as they expect the going will be difficult.  Speaker of the House, Paul Ryan, is a notable example.

Challengers also will strategically weight their odds before jumping into the fray.  If a sitting president is unpopular, or if the economy is not working to benefit the average voter, challengers have issues they can press as part of their strategy to gain elected office.