Many couples who use sunset clauses in their prenuptial agreements do it because they essentially want to test the marriage out. If it doesn't work and that becomes quickly apparent, they both want to get out of the marriage with all of their assets intact and with as little trouble as possible.
However, if the marriage does work and continues on for years, they want to revert back to a "normal" arrangement and do away with the prenuptial agreement. A sunset clause is a clause that sets an expiration date for the agreement. For example, after a decade, it may no longer be valid. The clause can be set for as long or as short of a time as the couple desires.
Couples do this because they assume the test will be over after that many years. They'll know the marriage worked, they won't get divorced, and so they won't need the prenup.
While that may be true, sunset clauses can be dangerous. After all, if you set your sunset clause at 10 years and get divorced after 11 years, you could lose a lot because of that one extra year of marriage. Plus, there's no legal reason to get rid of your prenup anyway, even if your marriage is working. You don't have to use it if you don't need it, but it can be helpful to leave it on the books just in case you do.
Before drafting a prenup in Texas, it's important to look at all of the clauses and options you have with a family law attorney and mull over the ramifications.
Source: Chicago Tribune, "Is a prenup expiration date an ex-wife's best revenge?," Margaret Littman, accessed May 20, 2016