The problem with egalitarian marriages is that every unpleasant task you have to do comes with the slight undercurrent of resentment that your spouse left you with the dirty work.
In fairly traditional marriage where roles are relatively defined, there are pleasant and unpleasant tasks that come with your role, but it is just part of the job. If your spouse steps in to help you with some unpleasantness or something you just can't do right now, the action is met with gratitude that your spouse is lifting your load.
When every task, pleasant or unpleasant, is open for a negotiating free-for-all, every task you do is because your spouse did not do it. This means when you do something unpleasant, it is directly because your spouse did not do the unpleasant thing which can feel irksome. When your spouse does do the unpleasant thing, the response is not necessarily gratitude because it was part of the expected job anyway. Everything is your spouse's job so nothing your spouse does seems above and beyond. Everything is also your job so there is nothing you can let go. The temptation for score keeping must be higher when everything is always up for grabs.
Nota bene: This observation was not generated from any particular real life event.