For each cleared cachestore in the plan cache SQL Server reports a message in the error logs , such as :
SQL Server has encountered 1 occurrence(s) of cachestore flush for the 'Object Plans' cachestore (part of plan cache) due to some database maintenance or reconfigure operations.
1) When the plan cache clears , a recompilation is forced , this can cause a temporary performance decrease
2) Using DBCC FREEPROCCACHE clears the cache .
3) Restoring a database and attaching\detaching a database causes a plan cache clear for the SQL Server instance.
4) Using DBCC FREESYSTEMCACHE clears the cache.
5) AUTOCLOSE = ON which causes the database to shutdown forces a clear down of cache.
6) Buffer manager sql counters and sys.dm_os_memoryclerks are good starting points to identify pressure