Flaw in the protocol / The introduction of a much better technology etc.)
A flaw in the protocol is unlikely, but it could also trigger me to leave.
I would be forced out of bitcoin, if there it is flippened by another currency, with better applications and utility (and increased adoption), I don't know if this counts as a better technology.
But if BTC is usurped in terms of market capitalization, value and adoption, I would switch to the other currency. (bitcoin would stiil be an alternative).