That is happening because when you cast you tell the service (YouTube, Netflix, whatever) to push the content directly to your Chromecast. It is not going through your laptop. Your laptop is just a smart-remote that is getting updates.
When you are done casting, go back to the cast choice and select "disconnect". Sometimes, if things get confused, you may have to actually select the device to reconnect to it before you can tell it to disconnect.
If all else fails, you'll have to unplug the power to your Chromecast to get it to drop the stream.
