That is very strange. I work from home and office. I have my system turned on all the time at home, ofcourse the monitor goes into standby mode after 10 minutes, but I have never noticed that my Annihilator 2 Ultra gets hot.
Then it also depends how well is your cooling and air circulation inside your system.
Also, I would check on the I/O Voltage. That can be a killer to your card and any other device in your system.