There’s nothing stopping them if they think the election was rigged – we saw that in 2016 with a couple of electors.
“Another scenario could involve “activist” state electors. Voters within the electoral college do not necessarily have to vote according to the popular vote within their respective states. Some states have laws that bind electors, but many do not, including Pennsylvania, Georgia, Arizona, and Minnesota. The states that do have laws have little ability to enforce them, and activist electors can only be slapped with a misdemeanor. Basically, there is nothing anyone can do to stop the electoral college from voting for Trump instead of Biden.”