Materialized View: What is the Update Trigger property?
That's the way materialized views work. They essentially create a copy of the results of the query in the view. The base table(s) used in that query  are automatically updated when the base table changes. That is implemented via a trigger on the base table that records all changes into a materialized view log associated to that base table.
A subsequent process then reads that log to apply the changes to the materialized view table. That application can be automatic or manual.
This: https://oracle-base.com/articles/misc/materialized-views has a nice illustration of how this works: