Mercury starved, it made a little over 3 years I think.
The phosphor is also turning grey
The other one I bought at the same time still works fine though
I have a Ushio CFL and it's DOA. A restaurant near my job had installed a dozen or so of them about 3 years ago...they all became mercury starved and glowed dim red, some after just a few months, but they were left alone anyway.