If it was a WordPress site with an active RSS feed that displayed complete articles, it only takes about 5 or 6 lines of code to tap into an RSS feed and automatically display the articles on another site. They could then run it through a content scrambler or spinner. This is all easy to automate. 40 to 50 lines of code in a single script would do it all. A cron job or scheduled task could run the script at regular intervals.
20% of your traffic from one competitor does seem suspicious. I would probably have blocked the IP addresses.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin