YouTube lets you pick the playback quality to suit your internet situation. If you’ve got a reasonably fast connection, it will start off at a lower resolution and automatically ramp it up to the highest possible setting based on your connection speed. You could also do this manually and the video streaming service has now switched up the order in which the resolution options are listed in the quality picker.
YouTube has simply inverted the order in which the resolution options are listed in the quality picker. The list was ascending, previously, with the options starting at 144p and going up to the highest available resolution.
It’s exactly the opposite now. The list is now descending which means that the highest quality is listed in the first spot followed the lower resolutions below it. This is a minor change that not a lot of people will pay attention to, certainly not those who have good connection speeds that are enough to automatically have YouTube streaming at the highest available resolution.
There may not be a particular reason behind this change. It does make sense to have the highest resolution option up top since most people would immediately be going for that anyway.