Due to a bug with Chromecast, you now see a phone app via your laptop instead of casting. Back in December, Google officially made a YouTube app on Chromecast that can be controlled with a remote on your phone, but a bug is causing Chrome desktop users to see that UI too.
Chromecast
The bug has been known since January and causes Chrome on the desktop (it doesn’t matter if it’s on Windows, Linux, Chrome OS or Mac) to cause problems. However, it only occurs in people who have a Chromecast that runs on Cast and not on Google TV. If you are on the YouTube website, you can conveniently cast the video to your dongle, using the app instead of the video playing immediately.
You are ordered to use your phone to pick out a video. Very useful in itself, but not if you use your laptop and perhaps don’t have your phone nearby at all. So very irritating. Apparently the software thinks that you are now looking up a mobile phone experience every time, while that is not really the intention. All you can do is try again: so close the Cast session and start a new one. You can, however, simply take control of your stream from your browser.
cast bug
At least, it’s not completely ‘normal’, because something opens that is clearly a phone-like app. There are several buttons on it that you can do nothing with as a desktop user. If you have Google TV, then you don’t have this problem. It is unknown if Google is already working on a solution or if it is close.
– Thanks for information from Androidworld. Source