Excellent observation! You've hit on a key, yet often undocumented, feature of Android. You are absolutely correct that Android has automatic mechanisms to clear cache, and it primarily kicks in when available storage drops below a certain threshold. The behavior you're describing is managed by a core Android service often called the Low Storage Space Manager or part of the Storage Manager Service. Its primary job is to prevent the device from running out of storage completely, which would cause system instability, app crashes, and an inability to save files or install updates. While Google doesn't publish the exact, universal numbers (as they can vary slightly by Android version and manufacturer), through teardowns of the AOSP code and widespread user experience, we know 2 general thresholds: Warning Threshold & Critical Threshold. The latter is the threshold you've encountered. It's typically around 10% or, more commonly in recent versions, a fixed value around 1GB of free space. This is the trigger. When free space drops below this critical line, the LowStorageSpace service is activated aggressively.
