Depends on what defines "stolen". Some felt Trump "stole" the last election, but didn't base their opinions on accusations of electoral fraud. They instead held a notion that an election could be stolen by improper or unfair influence - false things promoted, true things obfuscated. Today's unrest has many root causes; seen a certain way, events of the last four years could really piss off some of Trump's supporters. Imagine the people at the extremities of that dissatisfaction. It isn't necessary to excuse them to understand them.
The last straw is not the only one. It is amusing to see so many public figures racing for the highest seat on the horse; no one is to blame but Trump. Apparently there is some unforgiveable threshold of provocation that, out of all the things said and done, only he crossed.