He has bought a lot in Tiger's neighborhood in Jupiter, FL and plans to start building soon. He'll then move after his son graduates from high school next year.
If the estimates of his $40,000,000 income are correct, he could be saving almost $5,000,000 a year in state income taxes as Florida doesn't tax income. Realistically, if his "net" taxable income is only $10,000,000, he'll still be saving over $1,200,000 per year in state income tax not counting any local income tax, if applicable. I used two different calculators (one that I use for business) to arrive at these estimates. It looks like a no brainer to me.
https://www.morningread.com/in-other-golf-news/feature/2020-01-15/phil-mickelson-plans-to-follow-tiger-woods-to-florida