[Fido969]

A lot of those retiring will be replace by even leftier loons.

[Alberta's Child]

Some of these may also represent districts that will be carved out of existence after the 2020 census.

Also ... It's not unusual for Congressional incumbents from the majority party to retire before midterm elections. One of the reasons for this is that they know their own party mismanaged and botched their own agenda for the current session in Congress and is likely to get repudiated by the voters during the next election cycle. I suspect this is exactly what precipitated the mass retirement of GOP House members in 2018.