any plant can be purple under the right conditions.
Underneath the green chlorophyl, you have some other photosynthesis system which is purple.
In the same way that when your aerobic respiration fails in exercise, you have to go to anerobic, plants have a similar system.
Basically, it is a secondary back up system which is not as efficient.
I guess that purple plants are just ones with the genetics to lose the first system easily.