A new WSM will have air gaps at the bottom and top of the middle body ring. After you use the smoker a few time it will seal much better. Once you got the WSM hot it became a chimney and the hot air continued to rise and draw air from the bottom. With the outside temps at 25 and the inside WSM temps at 375 you have created a nice draft that was hard to stop. You will find out the WSM reacts to out side temps. I love my WSM an so will you. Don't be discouraged, keep on smoke'n.
Red Mastertouch,OTG Simpson,Blue Weber Date Code A,OTG,OTS,WSM 22,Char-Griller Pro