Why was he picking on me?

I have been taking cross fit classes at my sister in laws gym for a couple weeks now and the guy who teaches it he is 28 and he is or was a sergeant in the army. Well Tuesday he was having us do this exercise and i have never done it before but it was just a kinda different way of standing up then going into a lunge and then back down on your back but for the life of me i couldn't get how to do it i kept crossing my legs and he was laughing at me and asking me why i keep crossing my legs. He wasn't being mean i was even laughing along with me a couple of times but he was saying he doesn't understand why people cant get this workout and that all of us need to do a fundamentals class. It just felt like he kept picking on me. He corrected my squats and my sit ups and my push ups.