What could be causing waits on enq: TM - contention?

12.1 on AIX

This is small table, not partitioned, only 3 columns: NUMBER, VARCHAR2, NUMBER. The table doesn't have foreign keys and it isn't a parent in foreign key relationship. SELECT FOR UPDATE experiences sporadic waits on enq: TM - contention. I am pretty sure that DDL statements that may cause TM locks (ALTER TABLE, CREATE INDEX, etc) are not being executed. INSERT with APPEND hint, SQL Loader in direct mode also not used. What else could cause TM locks?