This happens to me on occasion but it's actually due to Gay-Lussac's law. When the door is left open, the inside of the freezer (including the air) warms up a little and expands. When you close the door, the refers kick on and cool everything down, causing the air molecules to contract and cause a vacuum effect. There's a mechanism that is supposed to equalize the pressure but it's certainly not the fastest thing in the world. I'm a big guy and I have to really shove to get the door open if it was recently open for a few minutes.