Because nighttime ambushes are fun for the DM! Plus, I don't want to give you the easy challenge of only one CR-appropriate encounter per day, and you don't want to have an over-CRed fight at level 2.
I just made sure that you wouldn't have to remember that rule by having it happen at the start of the night. Plus, it makes sense for reasons unknown to you that it happens now.
EDIT: oh yeah also what Z said! I want to see you two's reaction of waking up to a thornwolf assault and a random dude casting spells next to you, so far, with no clear side (I mean, Obscuring Mist
is a defensive buff for you and the wolves too - nothing is telling you that he is on your side...)
Ok now I'm just sowing discord.