At startup, Hardware sets the fans to maximum. As the software reads the temperatures of various components, the fan speeds may be adjusted downward if appropriate. A software crash will stop resetting the fan speed, so the Hardware will revert fan speed to maximum to avoid a meltdown.
In a running system, if temperature measurements remain high, fan speed will not be adjusted downward. I expect this is your problem. There are two common causes:
1) Dust buildup inside your Mac in general and in the graphics card fan in particular. this increases heat buildup and makes the fans run faster.
The approved method for removing dust involves Blowing the dust out of your Mac and into the air. Do this on the white carpet in the Living Room, and you are going to be sleeping in the doghouse for quite a while. Regular plastic vacuum cleaner wands and tools generate huge electronics-killing static charges, and should not be used inside your Mac.
2) Failure of any of the internal sensors: on the graphics card, a memory module, a processor, or in the PCIe slot area. Apple Hardware Test can tell you whether a sensor has failed.
