Headlines are sampled randomly for the first few hours of an article going live to measure exposure. The headline that gets the most clicks wins.
There are a lot of sites that do this.
It causes headaches when it comes to social. Usually the original headline is preserved in the url, but sometimes they'll use a unique id and then include the editorialized headline option so they can track which headline you clicked on.
Also editorial decisions on wording based on pushback, legal threats, etc.
What is it with Islamic terrorism and symbolic anniversaries? Is it part of a conscious effort to push memorial activities out of the news cycle? Or is it purely to spread terror?