As quoted in the Miami Herald article linked above:
In a letter to lawmakers, Scott said he vetoed the the alimony bill “tampers with the settled economic expectations of many Floridians Who have experienced divorce.”
“Because the subject matter of this bill involves family relationships, numerous Floridians have forcefully expressed their views on the topic,” he wrote in a letter released by his office Wednesday night. “Many Florida families have been impacted by the difficulties of marital issues, both concerning children and starting over.
“As a husband, father and grandfather, l understand the vital importance of family. In weighing the issues associated with this bill, however, I have concluded that I cannot support this legislation because it applies retroactively and thus tampers with the settled economic expectations of many Floridians who have experienced divorce.”