why select count(*) consumes wal logs

PG 10.5

I loaded 133 million rows to a wide table (more than 100 cols) via COPY. The table has
no index at this time. Since I am the only user I don't see any other activity.
Now when I run select count(*) on the table where I just loaded data, it runs for ever, 
more than 10min and still running. Intrigued, I checked locks and saw nothing.  Then I noticed something
strange.  When select count(*) runs, PG is writing to wal_logs, and that too a large amount. Why?  
I suspect vaccum is getting triggered, but this is a brand new table with no updates. So it should not.

Is there a SQL to peek into what PG is doing to write so much to WAL logs ?