Why can supplemental beta-carotene have deleterious effects?

O2 is a reactive oxygen species. Systematically reactive oxygen species can have deleterious effects. Increasing reactive oxygen species systematically is most definitely not a goal.

Beta-carotene is a carotenoid that produces O2 from singlet oxygen. Beta carotene can have quite high bioavailability. Supplemental beta-carotene through having quite high bioavailability can increase systematic O2 levels resulting in deleterious effects.