He started doing the YES thing when he turned heel and won the World Heavyweight Championship. He was a chickenshyt heel (although inside the ring he would kick the fukk out of guys like Big Show
) and as an annoying celebration, he started screaming YES at the top of his lungs every time he won a match and retained his belt. He took it from UFC fighter Diego Sanchez and modified with the hands gesture and jumping up and down in the entrance ramp. He was a pretty damn great heel so he started getting over and when the most hardcore fans went to Wrestlemania, they started doing the YES thing along with D-Bry (that shyt exploded and became hugely popular de night after Wrestlemania, when 12,000 kept doing it to show their support after Bryan lost the WHC in 18 seconds).
The NO thing began when WWE decided to keep D-Bry heel despite being so over after Wrestlemania. So, to stop people from chanting along and doing his shtick, DB went from YES to NO. He then formed a tag team with Kane and "NO" became his staple until May last year when he went back YESing and his singles push started.
Why is he so popular? There are a number of reasons. First of all, he has a VERY vocal hardcore fanbase from his indy days, even when he was a nobody in WWE there were chants for him (when he was fired, there was a PPV that closed with the crowd chanting his name). He started winning the rest of the audience thanks to his wrestling ability - he never has bad matches and since May last year, he became a the greatest "hot tag" wrestler in decades, people love to see him fly around and kick people at full speed - and he won the kids and casuals over when he was tagging with Kane. He became a lovable comedy guy for the kids, with a very distinct look, and he showed the casuals he was way more than a "good worker" in the ring. Add to that the fact that since he debuted in NXT, he's been a true underdog - sometimes in storyline and sometimes because WWE kept fukking him over for no good reason - and people kept rallying behind him thanks to being such a good performer, his catchphrase and wanting to see him get a legit run as the top guy in the company.
