this isn't a direct answer to your first question, but the people who go to burning man are probably familiar with the Rolling Stones album entitled "Get Your Y

this isn't a direct answer to your first question, but the people who go to burning man are probably familiar with the Rolling Stones album entitled "Get Your Ya Ya's Out" and since "ya" is a cry of agreement often used as a cheer, I always assumed it meant "group shout in celebration" and I don't think that's a big stretch as to your second question, humans have innate senses of violence as a reaction to certain stimuli, and also celebrations of victory whether in actual or symbolic combat so... can you seriously not see the emotional catharsis, or are you simply questioning it as being emotional and un-rational? I would add that to me--anthropomorphizing a little, emotions are rational: emotions were discovered by evolution as a good way to regulate behavior, and evolution is actually one of the most logical of processes.