1. Suicide
Suicide always strikes by surprise. No one ever says "I knew he was going to kill himself, I'm surprised it didn't happen sooner." The person who commits suicide is, almost by definition, a person who doesn't reach out to others or share their desperation. It's this self-isolation which creates the mindset that causes someone to believe that death is the only plausible solution to their problems.
(I have zero respect for people who commit suicide, but loads of empathy. If you are having thoughts of suicide, please reach out. In the U.S. you can call the National Suicide Prevention Lifeline toll-free at 1-800-273-8255.)
Cruise ships have two things in plenty: alcohol, and isolation (for those who choose to cruise solo). They also offer an easy method, which is close to hand 24 hours a day. In your land-lubber life you may not always be close to a bridge or on the roof of a tall building, but on a cruise ship you're only ever a few steps away from the railing.
This is a potent mix for suicide, which is often the result of a sudden impulse.
2. Crime
The combination of alcohol and isolation is also one that can breed crime. A rape, dispute, or mugging gone wrong could easily lead to the victim being knocked unconscious and pitched over the side.
3. Accidental Falls
Cruise ships are well aware of the risk of people falling over the balcony. It's virtually impossible to fall over the railings, most of which are at least 3.5 feet high. Unless you get drunk and decide to re-enact the famous "I'm flying" scene from Titanic, it's unlikely that people are falling off cruise ships accidentally.
4. Cruise Ship Serial Killer
This is a possibility posed in the comments on the Daily Mail article. Cruise ships would seem to offer the perfect hunting ground for a serial killer who's willing to specialize in burial at sea.
Two things work against this theory: first, most serial killers' victims are young, white women. Whereas the people who go missing from cruise ships tend to be older married people. Second, serial killers are actually incredibly rare, despite what movies and television would have us believe.