The less you weigh, the lower your BMR is likely to be. I would attribute that drop to your weight loss, and it likely would have been greater if you weren't strength training and building muscle (which helps to boost your BMR).
"You may have a fresh start any moment you choose, for this thing that we call "failure" is not the falling down but the staying down." Mary Pickford
"No matter how slow you go, you are still lapping everybody on the couch."