I tend to think it was Michael even though we didnt see it but, what do you guys think?
No, the chickenhead locked herself in there by accident. I believe that scene happened on purpose to build up the tension to her getting choked by Myers 
I think it was Colonel Mustard in the kitchen with a rope. 
But its strange though because the door was closed with force like if it was a ghost that did it…
It was an excuse for an booty shot. 
I was always thinking around that time "what the hell friggin song is she whistling!’
We already got glimpses of Micheal’s apparent supernatural side, he vanished right before Laurie’s eyes when she saw him looking up at her among the sheets, then we have the laundry room door than seems to close as if unseen hands closed it and then the opposite happens later in the Wallace’s kitchen when Bob hears, and we see, the door creak open on it’s own so who knows.
It’s his night. He’s been playing tricks already. He knocked the planter potter down and vanished. Killed Lester. Of courses it was him. He’s lurking right there in the shadows. 
Ditto!!
Aaaahhhh, yeah. In a blooper in the footage I have you can see Nick pull the door shut then he waves at the camera through the door…all this while in Myers get up. Pretty nifty.
C’MON Guys, That song was " My Paul" a Huge Hit in Haddonfield! 
This made my day
And then he got into the locked car. I think the fact that the door locked her in was interesting as well, a lock would keep others out not someone in. I always thought it was a subtle hint of the supernatural myself, there seemed to be sporadic amounts of that just to make things feel “off”.
I think it’s better leaving it up to our imaginations, could have been his hand, maybe the wind, maybe the little shack was at an angle & the door (via gravity) closes by itself if not opened wide enough and it was in the locked position when it closed (i don’t think she looks to see if the doors locked or not) she just tried yanking on it. I am glad they didn’t start teleporting him. Clicko turn off time if so.
About the car, maybe the passenger side door was open of the car & he went around that side & got in then unlocked her side to make it easier for her…
There are tons of ghost like supernatural elements to “The Shape” in the first film that set it apart from the rest. Myers vanishing from the hedge, from the clothes line, popping through the window unseen, etc., etc., etc.
Not to mention seemingly disappearing right before Laurie’s eyes in the neighbour’s backyard.
C’mon, Josh…He just did a quick sprint the length of the hedge…The clothes blocked her from seeing him simply walk off & i saw his foot as it was the last thing to go thru the window.