If you Google the Biggest Loser study, you will see an example of the one that has been most widely documented. Losing a tremendous amount at an insanely rapid pace can increase hunger and slightly lower metabolism. Worse, in the follow ups years later, most of them had returnwed to their old weights and still had low leptin levels and the slightly reduced metabolism; it didn't "reset".

Another cause is bad luck, probably genetics. It is harder for some people to lose weight, but it is possible for everyone.