You did not define a distribution style when you created the table you are loading.
As a result while loading the data Redshift is trying to determine based on the characteristics of the data (volume, attributes, ordering etc) Redshift will determine a best practice distribution style.
See this conversation for more info:
https://aws.amazon.com/premiumsupport/knowledge-center/redshift-fix-copy-analyze-statupdate-off/