Much simpler -- he thought he was so smart that he didn't need checks, balances, or guardrails. 

The Torah says a king can't have too many wives, less it distract him from his duties; Solomon said "I'm so smart, I can have wives and not get distracted!" -- and he got distracted. "Don't have too many horses, as that will cause the Jews to relocate to Egypt." *I'm so wise, I'll do the horse thing and still figure out a way to avoid a reverse exodus!* That didn't work either.

Your question of too much of a good thing is interesting in the broader sense, but there's a very clear lesson on where King Solomon went wrong -- he was so wise that he thought rules didn't apply to him.