When you're taking a photo, your guard goes down completely. You're focusing on your smile, trying not to make a silly face, and staring directly into the camera without blinking. This is the perfect time for a surprise like this to happen, as the mother in this video had no chance to notice the person running up behind her.
The young man is her son, a soldier who was serving in Germany. While the family was on vacation, he coordinated with his family to show up and surprise her while the picture was being taken. Little did she know it was actually a video, and this precious moment was going to be caught on camera for the rest of their lives.