This represents the sociocognitive theory of hypnosis.
This theory holds that we will behave according to what we saw other people behaving like. So, according to Brandon's friends and this theory (proposed by Edwin B. Holt in 1931), Brandon behaved as if he were hypnotized simply because he knew what other 'hypnotized' people behaved like - that is, he wasn't truly hypnotized but rather lived up to everyone's expectations.