Do you know when this started happening? Have you done any mods in the rear of the vehicle, have you been hit recently, was there any body work and/or mechanical work that may have required touching the tail lamp and/or related components?
If not, I would suspect that you're just unlucky and the LED unit is starting to fail. In which case you will need to get a new tail lamp assembly, unless you feel like finding a way to open the tail lamp housing up, determine what component on the circuit board/LED board is bad and manually de-solder/re-solder it and see if that fixes it. But I don't know anyone that has done that successfully, but it is worth a shot if you are going to replace it anyways.