Dental Disease is sometimes genetic. Rabbits with flat faces, such as lops and Netherland Dwarfs, have a higher chance of getting malocclusion genetically.
Malocclusion can also be caused by head trauma. Falling, chewing on wire bars, or getting into a fight with another rabbit can all break a tooth and create a large chance for malocclusion to set in.
An improper diet is the biggest culprit of dental disease. Rabbit teeth need to constantly wear on the other teeth and on substances like hay or wood, to stay their proper length.
Malocclusion can also occur easily in older rabbits, those over 5 years old.
"Dental disease can also develop as a rabbit ages. With advancing years, rabbits (like all of us) tend to lose bone density. When this happens in the already-delicate bones of the skull, the teeth can become ever-so-slightly looser in their sockets, and this can cause uneven wear." - excerpt from http://www.bio.miami.edu/hare/dental.html