Cassandra: memtable_offheap_space_in_mb and memtable_cleanup_threshold
Memtable flush happens in 3 ways1. When memtable_offheap_space_in_mb size is exceeded.2. When commitlog_total_space_in_mb size is exceeded.3.When forcefully Nodetool flush commend is executed. In your case I'm guess for second reason that flush happened. For more you can check this. http://academy.datastax.com/courses/understanding-cassandra-write-path/understanding-memtables-and-commitlog