вот еще дофигище возможных причин:
The whole procedure cache is cleared when certain database level operations are performed in the following scenarios:
A database has the AUTO_CLOSE database option set to ON. When no user connection references or uses the database, the background task tries to close and shut down the database automatically.
You run several queries against a database that has default options. Then, the database is dropped.
A database snapshot for a source database is dropped.
Note Database snapshots are only available in Microsoft SQL Server 2005 Enterprise Edition.
You change the database state to OFFLINE or ONLINE.
You successfully rebuild the transaction log for a database.
You restore a database backup.
You run the DBCC CHECKDB statement.
Note This is true only in versions of SQL Server 2005 that are earlier than SQL Server 2005 SP2. After you install SQL Server 2005 SP2 or later versions, the whole procedure cache is not flushed when you run the DBCC CHECKDB statement.
You detach a database.
You specify one of the following options when you run the ALTER DATABASE statement:
OFFLINE
ONLINE
MODIFY FILEGROUP DEFAULT
MODIFY_NAME
MODIFY FILEGROUP READ_WRITE
COLLATE
MODIFY FILEGROUP READ_ONLY
READ_ONLY
READ_WRITE
The whole procedure cache is cleared if one of the following server options is changed by the RECONFIGURE statement:
cross db ownership chaining
index create memory (KB)
remote query timeout (s)
user options
max text repl size (B)
cost threshold for parallelism
max degree of parallelism
min memory per query (KB)
query wait (s)
min server memory (MB)
max server memory (MB)
query governor cost limit
Note Procedure cache will not be cleared if the actual value does not change or if the new value for the max server memory server option is set to 0.
The whole procedure cache is cleared when certain database level operations are performed
