After upgrade to 19.5, a statement that runs 'normally' in 12.1 and 18.5 now is running with what I

After upgrade to 19.5, a statement that runs 'normally' in 12.1 and 18.5, now creates multiple sessions on the database and appears to be running in parallel.

The execution plan indicates this.  I didn't change any parameters between the upgrade from 18.5 to 19.5.  Any idea why it would behave like this now?

Is parallelism turned on by default?

I found a command to run - alter session disable parallel query, and now the statement runs in one session, and takes about 4 minutes compared to 8 with all the extra sessions running in parallel.

I need to make the behaviour work like it did in 18.5.  Does anybody  know what changed?