hey man my plants doin the same thing...its the end of the year and the plant knows its gona get smoked so its sucking up the rest of its sugars and thc and what not from all the fan leaves.. then in turn the fan leaves themselves turn yellowish then brownish then just die and fall off sometimes leaving its purple/dark purple or even black stem. you can either leave them on or help out and cut it off. i leave it on just so it gets everything it needs you know what i mean...once its all shribbily i cut it off. hey good news tho . your plants almost done.