If you read the description of the spell it normally starts as a cone 5 feet long and 1 foot wide doing 1d4+1 per your level. It spreads out as soon as you cast it from your hand. So for you that means a 60 feet long by 13 foot wide cone doing 13d4+13 points for a possible 65 total points of damage.
That's a normal Cone of Cold. Now lets double that...
120 feet
26 foot wide
26d4+26 points of damage, total possible damage 130
Now the doorway where the Goblins came out of is roughly 14 foot wide leaving a 12 foot excess, 6 foot on each side as the cone hits and attempts to fully pass through the door.
However the cone is not fully formed having only traveled at that point 18 foot from where Sirene is standing as the hallway is 20 foot wide and she is by the back wall so it is only 18 foot wide at that point. That leaves an excess of 4 foot, 2 foot per side of the cone that is NOT passing through the door. Instead that excess is impacting the wall at a high level of power. The initial impact is like a huge snow ball hitting a wall and exploding in all directions including back towards the thrower.
Unlike a snow ball this is a steady cone of cold made of ice and snow that hits like a hose of water and rebounds back towards anyone standing close to both sides of the door and moves down the wall for a short distance. This effect lasts 1 round only as the spell is instantaneous.
Wow that's a mouthful huh

but it's good you commented on that Nimu as itr does play a major part in my post and now I won't have to explain why what happens, happens. At least I hope not
(crosses fingers)