I'm going to go full autismo on this one.
I doubt they've ever seen snow before, so that probably had something to do with it. Slowing movements, disorienting them, etc.
Their first attack was a sneak attack, and Jack probably didn't even know he was outnumbered 7-1 at first.
Lastly, they probably never expected Jack to launch a sneak attack. He was pretty straight forward in confronting enemies in the past...future...fuck.
They were also down one member and throwing their rhythm off.