The R9 290 series cools different than earlier cards. You pick a target temperature and a maximum fan speed. The card then lets the GPU reach that temperature, running the fan only as fast as needed to keep it there. If it can't do that with the maximum configured fan speed, then it downclocks the GPU to keep the temperature at the target.
Since you didn't specify your settings for any of this, I can only surmise that you have your target temperature and/or max fan speed too low, which is causing the GPU to throttle.