Understanding why they do it

It's fairly easy to understand Donnie Placenta's conduct once you realize that

•  he considers being the so-called precedent (sic) a great opportunity to recapture the TV stardom he thrived on in his reality show days, and
• he knows he'll constantly be the center of attention by creating chaos and doing something outrageous every day. 

Donnie isn't qualified in any way to perform the traditional duties of the job, so he remade the job into something he knows how to do (professional media celebrity).

Donnie's failing as a so-called precedent (sic), but he's succeeding as a reality show performer. He's unpredictable because he understands that as soon as the audience knows what's going to happen next, they lose interest and stop watching the show.

Understanding the actions of the Republicans in Congress is harder, but Washington Monthly has an explanation that seems to fit the available facts.