Yes, drafting. But the mind-blowing part you may see if you compare the speed and power between the two races. Same course, same basic conditions, a few hours apart.
I just guessing, but I'd say the 123 race was smoother with higher corner speeds. I'm saying that as you had a lower power and HR in that race but the speed was higher. I am jealous btw, it will be May before I can double up with 2 crits.