This happened partly because of establishment and leftist types normalizing violence and undermining confidence in the electoral process. You wanna crow about how your rioting is getting positive results? Don't be surprised if other people try it. You wanna scream "all cops are bastards"? Don't be surprised if other people turn on law enforcement. And if you want to insist Trump couldn't have won without foreign interference... well, don't be surprised if people insist Biden couldn't have won without fraud.
Like it or not, actions have consequences. This is the chickens coming home to roost. Am I trying to justify what happened? No. What I'm trying to do is explain that when you try to normalize a tactic or an idea, it will inevitably be used against you. Pro-slavery politicians justified their stances based on states' rights, then started whining when Northern states passed personal liberty laws to prevent the return of runaway slaves in defiance of the Fugitive Slave Act. Trotsky endorsed violence and terror against political dissidents, then bitched and moaned when Stalin persecuted his followers. Nazi Germany committed terror bombing against Polish and British cities, then cried foul when the Allies did the same thing to Germany in retaliation. Things like this have happened over and over again. And they'll probably keep happening for as long as humanity exists.