From the Dallas Observer:
https://www.dallasobserver.com/news/dallas-mayor-eric-johnson-announces-he-is-switching-parties-from-democrat-to-republican-17534050
For those who have been paying even somewhat close attention, this news, although still surprising, isn’t exactly the most shocking thing to ever come out of City Hall. For months, since Johnson won reelection after running unopposed in May, he has openly and rather boldly carried himself in ways that lean more red than blue. The former Democrat state representative for District 100 has made multiple appearances on Fox News, and in June, he invited Republican U.S. Sens. Ted Cruz and John Cornyn to the city’s 2023 inaugural ceremony.
In the WSJ article, Johnson says he was never a fan of the Democratic caucus during his House run, nor, according to him, were they a fan of his.
SMU political science professor Cal Jillson isn't as puzzled by the timing of the mayor's switch as some others. Understanding it is as simple as understanding the political party landscape Johnson is navigating.
"You can only be elected mayor of Dallas as a Democrat," he said. "And you can only be elected statewide in Texas as a Republican. I think that helps to explain the timing. After his election, he's now giving Republicans time to get their mind around what has happened and to get comfortable with him as he prepared to think about what office he wants to run for."