Edit: added a bit of information below for those interested
A/B testing (also known as split testing or bucket testing) is a methodology for comparing two versions of a webpage or app against each other to determine which one performs better. A/B testing is essentially an experiment where two or more variants of a page are shown to users at random, and statistical analysis is used to determine which variation performs better for a given conversion goal
In an A/B test, you take a webpage or app screen and modify it to create a second version of the same page. This change can be as simple as a single headline, button or be a complete redesign of the page. Then, half of your traffic is shown the original version of the page (known as control or A) and half are shown the modified version of the page (the variation or B).
The following is an A/B testing framework you can use to start running tests:
It's likely A/B Testing and when people receive the new layout, if Youtube sees a drop in engagement for their experiment they may just ditch it.
Oh, thanks for the link!
Edit: added a bit of information below for those interested