I think you're right theflyinggeek, it is because of your BMI. Because a class 2 medical doesn't require a EKG, the CASA medical review board probably wants to ensure your ticker is ok because of your BMI. I'd take a stab and say that if it were a class 1 medical and CASA had those EKG results, they probably wouldn't have had to recommend you for the stress EKG.
Can only hope you're not going to upgrade next year to a class 1 and have to get a EKG done all over again....