Setting aside possibility of group thinking (with powerful echo chamber) or making a genuine error in judgement concerning quality of evidence?
(Yes, such mundane explanations usually work. I personally find impressive, that after flop of Mueller investigation they decided to double down and this time not only make an investigation but launch an impeachment so I no longer give them any benefit of the doubt)
However, if we want a more mastermind explanation, I have one:
why risk an impeachment move now when the public will vote on the next president in a few months anyway?
Let's me take some famous quote (out context ;) ):
"if we don't impeach this president, he will get re-elected."
Your question is based on implicit assumption, that winning an election is (relatively) easy, while impeachment is (relatively) hard. Is that really the case? If one track bets, people are clearly betting on Trump. So what if Dems looked at their chances in straightforward election were bad, so there is some logic in betting everything on one card and hoping that impeachment would find actual crimes or at least dig enough embarrassing dirt on their main opponent.