Lock escalation is triggered when lock escalation is not disabled on the table by using the ALTER TABLE SET LOCK_ESCALATION option, and when either of the following conditions exists:

• A single Transact-SQL statement acquires at least 5,000 locks on a single nonpartitioned table or index.
• A single Transact-SQL statement acquires at least 5,000 locks on a single partition of a partitioned table and the ALTER TABLE SET LOCK_ESCALATION option is set to AUTO. 
• The number of locks in an instance of the Database Engine exceeds memory or configuration thresholds.
• If locks cannot be escalated because of lock conflicts, the Database Engine periodically triggers lock escalation at every 1,250 new locks acquired