@Potbelly (View Post)
Because the particles create pressure in their way and they start moving according to the pressure. It seems that either the top left corner stays with the least pressure or the oscillation time is very slow.
Cate's TL;DR:
The pressure map is decreasing pressure to 0 in the left edge. It's a map miscalculation.

The code that causes pressure to decay towards zero at the far edges of the screen is slightly closer to the inside of the field at the left edge of the screen, for some odd reason. NBLE gas always increases pressure or decreases it until it reaches near 3.5, and with the pressure becoming closer to zero on the left edge where air escapes due to this inaccuracy, it appears to bunch up.

 

Just make the walls thicker. :P