Seems to be because they can.
It seemed like a good idea in the really blue districts.
Because they smell some smoke there (it is not like Trump doesn't do anything worthy of scrutiny), and figure if they look hard enough they will find fire.

Early on, it looked to me that they were trying to bait Trump into an Obstruction of Justice charge. I think they were going there with the Mueller investigation, and had it all cued up for this one until Trump released the call transcript and Nancy "forgot her speech on the plane".

See you in November

It's about hate, power and pride

It's all about hate which is born out of pride and that being the first sin and the one that got Lucifer and all of his followers cast out of heaven. Pride is the locus of control in the world and nowhere on earth is it more prevalent than in DC.