Why does mds process heat up CPU after hard disk install?

I recently installed a new hard disk on a MacBook Pro Core Duo 2.0 GHz and migrated accounts from the older disk. The Activity Monitor has started showing the mds process ramping up suddenly and dominating the CPU usage at over 100% -- occasionally as high as 170%. Then the process dies down until something else causes it to ramp up again.

After two days without problems, this cycle has begun to repeat itself. Can someone explain why this would happen, and why so sporadically? Thanks in advance.