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.
There's also the famous midrash that he had a ring upon which was inscribed THIS TOO SHALL PASS, and that he'd put it on if he was feeling down ... or if he was feeling too happy. What's so bad about feeling too happy? Eli Wiesel z"l said that as long as there's something wrong, there can be hope for it to be better ...
(Again, not quite a "cause for downfall.")