Stock Watch: Can someone explain to me what those giant spikes are during after hours trading?

I'm a novice when it comes to stocks, so I was hoping someone here could help me understand something. I notice a lot of stocks I watch have huge spikes after hours, even though they don't really seem to really affect much. Can someone explain what is happening when this occurs?

For example, here is $AAPL's after hours yesterday. What causes this?