'Force Grid' Algorithm

[NGen]

In the game 'geoDefense', there is an effect that I'd like to call the 'force grid' algorithm. It's somewhat hard to explain, so here's a few images:

http://images.macworld.com/appguide/306804246/ss2.jpg
http://wirelessmedia.ign.com/wireless/image/article/969/969322/geodefense-20090402040921063.jpg

The squares in the grid in the background are being pushed outwards as the turrets fire, which can be used for some pretty cool effects other than what it was used for in those pictures.

The effect has actually been somewhat recreated in this program:
http://gmb-time.blogspot.com/2009/07/gravity-grid.html


However, I do not think that the above system supports multiple forces to act on the grid at once. Does anyone have any ideas on the theory of the force grid algorithm? I'm completely lost here.