My theory is Americans got sick of Open Contempt for Rule of Law.
This Shellacking was too widespread and diffuse across every state to be explained easily by the Usual bread and butter issues.
It points in my opinion to deep outrage about Aggressive War, War Crimes, and ICE Abuses. CONGRESS Enabled Deep Criminality by the Executive abetted by the SCOTUS.
This is the reason in my opinion.
The Statehouse Tsunami was also driven by exasperation with Supermajorities Comtemptuous of Citizens basic Rights.
Not the Usual things that motivate voters.
Americans have shown real Backbone.
Perfidy and Murder are real Crimes.
Lack of National HONOR is a Stain that doesn't fade.
The Corruption of Mind involved Repelled many people even if they didn't acknowledge it Conciously.
This,combined with high Costs, caused directly by these Crimes gave an Impetus to get the parties involved out of Power.
In a Parliamentary System Trump would already be gone.....
It's Easier to say High Gas Prices,but that's not entirely so.