I agree; probably not a direct expression of inherent genetic mutation. It is possible that environmental conditions (including those produced by any person(s) caring for the plant) caused a point mutation resulting in abnormal growth post germination; however these types of mutations are rare as well (at least ones that cause a significant change in the plant without killing it). I would say it is probably a factor of temperature, ***light***, and/or watering/feeding resulting in positive gravitropism (downward growth) above the root level. Usually positive gravitropism is seen only in roots, which grow down, and negative gravitropism is seen in stems (which grow up, obviously). The amount, type, and direction of light can all cause directional changes in growth, but don't necessarily indicate a change in gravitropism (specifically dealing with gravity). I'm just throwing out ideas, as it could be one or multiple factors that may or may not be related.