Because your Mac is a little old. Startail uses Quartz Extreme technology, that needs computation power. If your graphic card is too old, the work is done by the CPU. When the CPU load grows, the fans turn faster.

Startrail has been developed on a iMac G5 2.0 GHz, with an ATI Radeon 9600 graphic card. With this configuration, fan acceleration has been experienced only few times. On older machines it could happen more often or even continuosly.