The employer may take on certain expectations, possibility choosing a hire based on that enthusiasm and apparent commitment. Either way, no one expects a new employee to a) dislike a place soon after coming onboard, or b) abruptly take another offer. Is 12 months abrupt? Depends on the expectations.
It's a difficult situation that has to be handled tactfully on both sides. Doesn't appear to have happened here: Jason for his response, Evan for resigning _via email_.