Unwinding E string

I bought some Mega Slinky strings when I bought my guitar like 6 months ago. No issues, sounding great, but today as I was playing a riff on the high E/B strings the E string kept going out of tune suddenly, so I would get it back in tune and continue and then a few minutes later it would go out again. Nothing happening on the adjustment end. String sounds normal when re-tuned.
This continued until about half an hour later the string broke.. or I thought it broke until I looked at it, and what had actually happened is the end of it unwound from the metal circle thing on the end (attached to the string, not the guitar). The metal bit went flying.
I never thought a string would unwind itself from there. Anyone seen this before?