Particles are updated based on position on the screen, from top to bottom and left to right. The top-left most particle is first updated. Then the one to the right and so-on. Once the row is done, the row below that one is updated. Again left to right particle updates.
This is evident by not only the game's source code, but how SMKE and other elements have a left-bias in interaction (direction via pressure, etc). This save demonstrates that principle: