Multiple solids/shapes layers with a card wipe effect applied. Turn the back layer to off, transition width to 0 and a small "Timing Randomness"(change the Random Seed on each instance of the effect) to get the basics of the effect. Lastly you need some sort of gradient to get it to start from the center and move outwards. Then just play around with the rows and columns size until you reach the desired result. Check out this quick and dirty example project I made.
Used to be able to upload example files to the Cow, but I can't get that to work now---I made an example file you can download from Dropbox
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.