I think it's a couple reasons, but absolutely not bird flapping at all. Not once have I ever instinctually flapped. Just spread my arms out further and further. When jumping from 15+ feet or higher, you know that spreading your arms does two main things. It aids in balance, and sets you up to potentially grab/hit/get tangled in something and potentially slow or stop yourself. It also increases wind resistance thus slowing your terminal velocity.
At less than large heights, the above post is the most widely accepted answer.
Who the hell feels natural flapping? To the person way above who mentioned their baby waving their arms, that's called a tantrum, and flailing limbs is not uncommon.