What just happened?

That sounds pretty odd. Can you close your Internet Browser and reopen? I know perhaps it's a silly question but you could have some adware attached to the video you were watching on YouTube. If you can't close it, you could try to scan for viruses or adware. That's all I can think to suggest. Good luck and Brightest Blessings.

You may have some kind of malware on your computer that causes your browser to switch urls on you. It's weird that it would choose Youtube though. Have you run any anti-spyware or antivirus programs? Do you regularly scan for viruses and malware? I would suggest you do that, and if all else fails (including a reboot, or a total shutdown and restart) consult a technician, or if you feel you could troubleshoot yourself, try one of the PC help sites such as computing.net. I hope you sort out the problem!

Sometimes, you loose the connection to the internet and it happens. So, I don't think it is anything to worry about at all. I think sometimes it is a sign that one has been on the internet for too long and it is a way for the internet to tell you it is time to take a break.