I'm with orgopete, I'd say that over-oxidation is a likely culprit. Benzoic acid?
Not surprised about the aromatic substitution, I think I've seen chlorides + peroxide as a green-chemical chlorinating reagent before. I think with a copper catalyst? Maybe could reduce this if you made sure everything's clean of transition metals.
One other thing; peroxides make radicals, radicals polymerize styrene, or at least cause anti-Markovnikov addition.