This is what I think happened. For one thing, I always knew that Casey exerted as little effort as possible when she disposed of Caylee. I believe she killed her in cold blood, and with intent to do her harm, and then I think she wrapped her up in the bag and taped her up and threw her in the trunk. I believe the remains began to seep out of the top of the bag or in some rip that had occurred either when she placed it in the trunk or when she was getting it out and thus the decomp stain in the trunk. I think that she KNEW this area VERY well, as it was her "backyard" so to speak and she was insistent on the fact they should concentrate on places that were FAMILIAR to them...ie: OUR woods, the woods where we PLAYED, the woods where we may have partied as teens, the woods that we knew like the back of our hand. I believe she parked at Lee's and entered the woods with Caylee's remains and she KNEW the flooding patterns and she KNEW where the retention pond was, and she went THERE and dug Caylee a VERY shallow grave and perhaps even placed something, a rock or a paver? on top of her to keep her in place...and she left her there, out of sight, buried, possibly sprayed down with insect repellent to keep the "smell" down. I think she then walked away and did not bat an eye...
AND THEN
the rains came...and more rains came, and it was torrential downpours left and right and at the same time, as corpses tend to do, Caylee "swelled" and her body "rose up" out from under the soil and came, quite naturally, to the surface because of this swelling that occurs in a corpse, and perhaps even slipped out from under whatever was holding her in that spot. I believe the rains then displaced her remains from the original exact SPOT where Casey had actually DUG the shallow grave, and when the waters receded, she was deposited right there in the spot where she was eventually found.
AND
I think the Almighty had a little something to do with HOW she was finally located...sheer dumb luck...