By updating more than 20% rows in the table you are triggering a statistics update.
The default behavior of sql server is to do this synchronously. This means that if an index affected is used in an execution plan the optimizer notices that the statistics are out of date, forces an update right away and re-compiles the execution plan.
There is a new feature in SQL Server 2005 that will make the statistics update asynchronous for the database, i.e. the optimizer will use the stale statistics, but run the statistics update in the background, so that the next query had take advantage of the new stats. The command is
AUTO_UPDATE_STATISTICS_ASYNC ON see BOL for more information.
The alternative as already suggested is to force an update manually on the table after the load. You can do this with the
UPDATE STATISTICS command. Here is the BOL reference.