Obviously COVID-19. Trump was at 49% approval and things were looking great at the beginning of the year.
No, Trump has been behind in the polls [with various statewide and congressional results to back up those polls] with underwater approval ratings his entire term and was very likely to lose anyway. COVID just made it official.
He won at 38% approval and behind in the polls. He was now at 49% approval and the far majority of Americans per Gallup said they were doing better than they were 4 years ago. Record unemployment, record stock market, peace in the Middle East, things were looking great.