Assigning additional reviewers?

I have submitted a paper that I have written at a well-known journal and the current status is: Assigning additional reviewers. I have just submitted my paper last week. I am wondering why this is the case because from what I know, initially the associate editor assigns 2-3 reviewers and when they complete their reviews, if the editor feel that the reviews were not sufficient may assign additional reviewers. So why is my paper being assigned additional reviewers at the beginning of the review process? Thanks.