While starting up the remote it lit red and beeping.
I launched DJI GO, and it told me that the RC needed calibration.

Any ideas why?

The principle is the same as this DJI P3 Video:

terrylewis Posted at 2016-3-7 07:21
The principle is the same as this DJI P3 Video:
https://youtu.be/w6fCd2gWQzc

Thank you Terry for helping out other users!

I knew how. Just wondered why...?

Roger(NO) Posted at 2016-3-7 14:50
I knew how. Just wondered why...?

Ahhhhh, sorry! That's a much harder question

Just guessing, but since there's a calibration procedure, there must be a data table somewhere that gets stored with the RC potentiometer variables. For whatever reason, the GO App wanted updated data.

Perhaps a firmware update, or random write corruption invalidated your data. We'll probably never know for sure...