Turning down an accepted tenure track for a better one?
Because of the way the academic schedule works, if you accept a job you are expected to go to that job for an entire academic year.  This is different from other kinds of jobs where hiring happens all year round.  As a part of this, you are expected to withdraw your other applications when you accept a position.  Anything else will usually be seen as unethical behavior by most people.  This can get complicated in some corner cases (eg you take a temporary job but then get offered a permanent job, where the usual ethical move is to delay the start of the permanent job for a year, but often there’s no hard feelings if you just drop the temporary one, especially if you’re nice about it), but it’s the standard ethical rule.