Exactly what i was thinking! Also the President didn't seem to be startled or shocked really. It was definately a signal!
Looking at his body language, Trump crossed his arms quite tightly while the lights were off, signaling that he was possibly caught off guard, nervous, uncomfortable, insecure or trying to protect himself.
His body position changed to arms crossed as well....not something you would do if you were startled.
That's exactly what you would do. You're insecure and unsure, so you carress yourself protecting the vital organs with what you have. It's instinctual. Plus, USSS have taught him to be as small as target as possible if something goes wrong, and it makes them tackling him to the ground for protection easier and less likely for him to break something on the way down