Thad Cochran won reelection in 2014, and then retired last April because of health reasons. Most states have laws that say if there’s a Congressional vacancy that extends so many days (I think in Mississippi it’s 60, but don’t quote me on it) from the next time the seat is normally up for election, there needs to be a special election for whoever would hold the seat for the remainder of the previous incumbent’s term. So now that we’ve had the special election for the last two years of Thad Cochran’s term that began in 2015, we have the election for the full six years in 2020.
The same thing is going to happen with John McCain’s seat. John McCain was re-elected in 2016, died in August of this year, and there’s an appointee in the seat for now, Senator Kyl. Arizona does it a bit differently so that a special election got shifted off into 2020, rather than 2019, but I digress. But in 2020, there will be an election for the last two years of McCain’s term, then there will be an election for the full six years in 2022. Interestingly, that means that Arizona is going to have a Senate election for five straight cycles (2016 McCain full, 2018 Sinema full, 2020 McCain Special, 2022 McCain full, 2024 Sinema full)