The reasons for the EXCPDEF file needing to be updated on DC startup would be
If the data collector was started with either an empty exception data set, or an exception data set whose level was less than current.
If either of these conditions are true, the exception file will be updated during startup, either to populate an empty file, or to upgrade it to the current level.
If the exception file is current, then no writing occurs during startup.
It is recommended that the Datacollector Start Task id to be givend update access to the EXCPDEF dataset