It’s common in every election for the losing side to cry foul and claim there was fraud.
Investigations happen. They find a couple of bad votes. The losing side remains unhappy.
But for the losing President to do this is astounding. But Trump is known for perpetuating bad rumors and holding onto them for years.
Example: “Obama was not America but born in another country.”.
That wasn’t his first time either. He’s done that in other things too. It’s just.. never has he gotten so many to believe him.
For a sample, search for:
election fraud year
election fraud 2012
election fraud 1992
Pick any election year in the USA (any even numbered year).
You’ll find similar tales.