Depends on the woman. For some, anything can change it—stress, diet, exercise, sleep changes, intercourse, caffeine intake, drug use, getting out of bed on the wrong side…
Stress, weight changes, medications, pretty much what’s been written above, I can vouch for. If you want to trigger a cycle then just plan to do something wonderful on any of the following:
Holidays
Birthdays
Vacation
Planned romantic weekend getaway
Anything. Just thinking “huh, I hope I didn’t get knocked up last night” can make my period go into hiding, leading to stress, which makes it delayed. The stress leads to weight gain or loss, which makes it delayed… @nikipedia gave a great answer.