Innate Knowledge as a Violation of Causality
Innate ideas are said to be known without being acquired—this is an effect (knowledge) without a cause (acquiring the knowledge). For you to have knowledge you must have acquired said knowledge by some means.
After all, an idea is a form of awareness of reality, an innate idea is accordingly an awareness of reality prior to any contact with reality. There could thus be no causal chain linking the idea with reality and as such the believers in innate ideas must resort to supernaturalism.