You will often see this happen under a couple of conditions -
1) The mid-term election after a President is elected and has a Congress of the same party in charge.
Generally, those mid-terms go against the sitting President. Whether it's greater complacency among the party in power's supporters, since they have that power, or greater motivation from the supporters of the party out of power, since they really are in the back seat, this seems to be a trend.
2) Members who were part of a "wave" election where a particular party did well in areas they demographically struggle are up for election, often without the coattails of a more popular national candidate.
In both cases, you will find a high percentage of those announcing their families suddenly need them more than before to have had toxic polling numbers that suggest they would be ousted either in the primary, or in the general election.
When those numbers take that turn, national PACS will also notice, and the problem will be exacerbated by them looking to spend their dollars in more competitive or winning efforts.
Sometimes it is the person wanting to retire, but quite often a lot of the people retiring see either an insurmountable or uphill battle, and throw in the towel.
As far as "why not just run and see if you can't pull it out" -
If someone has been in office long enough that the stink of being in Congress sticks to them, the electorate that initially put them their might still be inclined to elect a new face from the same party, thus having a better chance of the party protecting that seat.
It's also easier to cash in (if I may take a cynical turn on the subject) on the office when you're lining up your ducks while still in power and still have influence that potential future "employers" want to access, than if you go trounced and are looking for a job at that point.
The next election is going to be a national one, so there will be a national candidate to attach oneself to. Probably a lot of the ones who would either not benefit from being associated with Trump, or would be unwilling to try and ride that wave have already dropped out on the GOP side, for House races, and a lot of Democratic candidates foresee continued motivation for their voter base in the next Presidential election, so I doubt you'll see the kind of pre-determined attrition that you do in midterm election cycles. Though, in this case, there will be a lot more Senate seats that went red in the 2014 mid-term that they have to defend in the presidential cycle, so that would be the demographic I'd look at, if one was looking for this kind of trend.