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The first 5 Mishnayot of the this perek all deal with situations in which the consequences of a spouse's vow are deemed so damaging to the marriage, that the Halacha rules that the marriage must be dissolved, and that the wife must be awarded her full Ketubah payment.
Although all 5 Mishnayot look like they are dealing with variations on the same theme, there is actually an important distinction between the first Mishna and the next 4. In the first Mishna it is the husband who is uttering the vow, and in doing so undermines a basic obligation of marriage that is incumbent upon him.
In the next 4 Mishnayot (despite the prima facie impression created by the Mishna's language) it is actually the wife who is making the dramatic and untenable vow. What's going on here is that the husband, who has the right to annul these kinds of vows of self-deprivation that his wife takes, is in these cases obligated to annul these vows, and is failure to do so creates grounds for divorce with full Ketubah. Why? Because the assumption is that the wife has utterred this vow as a means of testing whether her welfare and happiness are of real concern to her husband or not, and his failure to annul it affirms her suspicion that he is in fact indifferent to her welfare and happiness. This reveals the marriage to be untenable, and the Bet Din then orders that it be dissolved (via his giving her a gett.)
The distinction that runs throughout these Mishnayot between cases in which the husband is a Yisrael or a Kohen, is due to the permissibility of a Yisrael and his wife deciding to bury the hatchet and remarry should they choose to, and the prohibition upon a Kohen and his wife to do the same. We thus give the Kohen and his wife a little more time to work things out (should they be so inclined) before the divorce is ordered.
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