Mine was entirely spur of the moment. I hadn't been able to play anything on PC online for months and months so I decided to test if I could one day when my Xbox Live stopped working (it still doesn't work, actually). I had played as another entirely random name (am ur pokemans ok?) to joke with a friend of mine last time I could play, so I had to go in and change my name before I started to play. I guess I was just really tired or I wasn't expecting it to work, but I just decided to use a new name for this one test run. I was drinking some Yoo-Hoo and me, being the epitome of wit, decided to reference Soylent Green with my name. Thus, 'Yoo-Hoo is People!' was born. I just never got around to changing it after a while on the eGO servers and I didn't feel like losing the familiarity by changing my name, so I kept it.
This is the only place on the whole of the internet where I'm not known as 'Aurens' (not counting my GameFAQs account from 2002).