PostgreSQL: What are different operations make the use of Temp files?

We have time series monitoring on both pg_stat_database as well as pg_stat_statements. Recently We have been observing temp file usage in our DB during certain period. But for the same period we are not able to find queries that in total equates to total usage from pg_stat_database. We have seen somewhere maintenance operations like vacuuming and analyse can cause temp file usage, but for the period we couldn't find such activities as well. Is there any other internal operations which can cause temp file usage?