In Tillandsia, the point from which new leaf growth originates is called the apical meristem. The meristem contains groups of actively dividing cells which give rise to new tissue. Normally, the meristem is dome-shaped, leading to the predictable growth pattern we all know and love, wherein new leaves emerge always from the direct center of the plant.
Usually, when a meristem is damaged, it (along with it's Tillandsia) will die. However, a rare condition can occur when a meristem is damaged but continues to grow in mutant form: fasciation. Fasciated plants are often worth much more than regular plants, owing to their rarity and unusual beauty.
Fasciation can manifest as Crested growth or as Monstrose growth. When a meristem is damaged "just right," it can elongate into a line-shape, leading to multiple, linear growth points instead of the single predictable growth point we usually see. Monstrose growth occurs as bulbous, overflowing growth made of multiple and unpredictable growth points. Monstrose growth is not typically seen in Tillandsia but is common in cacti.
Besides physical damage, cresting can also occur from chemical exposure (to colchicine and more), genetic mutation, and more. Some crested Tillandsia known to pass the cresting trait to offsets include: T. ionantha 'Pachara Predator', T. ionantha 'Monstrose', T. exserta × T. fasciculata Crested Form, T. Amazing Grace (Bird Rock Tropicals), and T. xerographica × T. capitata (Tillandsia International). This indicates a stable genetic mutation.
Some species are more likely to crest for whatever reason, including T. ionantha and T. caput-medusae. Biologically, cresting can occur on all parts of a plant (shoots, leaves, flowers, roots). Outside of Tillandsia, Cockscomb plants are able to transmit this trait through seed. Plants with stable cresting often have the latin "cristada" in their scientific names.
Interestingly, there may be a connection between an infection of bacteria called phytoplasma and cresting. Phytoplasma are parasites of the plant phloem and might be responsible for an evolutionary change way back in the history of two plants that now are permanently crested (Tropical Monkey Ladders and a certain mistletoe).
The difference between normal, crested, and monstrose growth-same species. Photo: www.baetanical.com