If it happened all at once, they would be able to react and change. Because it happens slowly, over time, the process just sneaks up on couples. But if you put a frog in a pan of water at room temperature and slowly increase the temperature to a boil, he'll stay put until it's too late. In much the same fashion, the rut just sneaks up on couples. They stop communicating and slowly drift apart.
Lack of communication can mortally wound a relationship.
Let's look at how communication in a relationship can break down. It can happen in one of two ways. Sometimes, couples completely stop talking to each other in what you might call an extended version of the silent treatment. This is usually the result of some event or series of events that caused so much hurt and anger that the couple just shuts down communication. Although this can and does occur, it seems to be the rarer of the two.