What makes cakes sink in the middle after they come out of the oven?  I just made 12" cakes and the both dropped in the center. 6" was fine

Did you use upside down flower nails towards the center of the 12" cakes?  Cakes with more surface area need a little help baking evenly.

No I didnt. Great idea, I'll try it. I was wondering if I overfilled the pans.  Thank you! I wouldn't have thought of that 

Here's a tutorial with photos on using Wilton Bake Even strips and metal flower nails with cakes over 8".   I will print the title of the thread here in case the link to the wilton site doesn't work (yesterday CakeCentral wouldn't let a link work....)    If the link below doesn't work, just copy the title with quotes and google.   This site won't let me do a preview either.....sigh.......  I hate change.....

"Bake Even Strips and Flower Nail on larger cakes --Photo tutorial"

http://www.wilton.com/forums/messageview.cfm?catid=8&threadid=148262&FTVAR_MSGDBTABLE=

Yup.  Now that it's posted (with no edit button), the link won't work, so do the google thing.   Sheesh.

My cakes don't sink, but they shrink from the sides as soon as they come out of the oven. Is that normal? Would the baking strips help?

Sweetshop~~ Different recipes will have different results.  Usually a doctored mix with pudding in it will result in shrinking from the sides of the pan (normal).  Using Baking Strips helps the ENTIRE pan of batter to cook more evenly.    Sometimes a recipe will shrink from the sides, sometimes it will rise an inch above the pan while baking (use a parchment collar!!!!!!!), then settle down to a level 2" high cake when fully cooled.  Sometimes a recipe will create a dome, sometimes it will bake flat.

In other words.....PRACTICE PRACTICE PRACTICE until you KNOW your recipes and the characteristics of EACH recipe.  No easy fix here; just knowledge based on repeated efforts to see what happens.   

As we say, "You can always eat your mistakes".   (This includes cutting or spooning out the middle that started sinking while it cooled and turned into a gooey mess/cutting off the crispy and burned edges/cleaning your oven [AGAIN!] because a cake pan overflowed/having your gorgeous, 2" high cake sink to become a 1" layer when cooled - - - - - - - - the possibilities are endless.)   Simply spoon into a pretty dessert bowl and slap on some whipped cream with a cherry on top!  WAH LAH!  Fancy dessert.