Loud fighting on father's day.
One thing I havent' seen on these threads (tho I can't keep up) is the father question. And the agency of Caylee. Caylee was at an age where she would be interested in who her father is. After being stimulated by a "father's day" event, she'd come home and be asking Casey, Cindy, George to explain about her father. She was probably relentless, like a two year old, full of "why" and "what was he like?" and "did he love me" and suchlike things.
The story Cindy gives, that the father died in a car accident when Caylee was one year old, and that she didn't remember his name, is the kind of a story that you would give a toddler. (Certainly not a story that you give the police!)
If Casey wanted to maintain a peaceful fiction about the father of Caylee, she must have been angry about Cindy stimulating Caylee's interest in her father by that father's day trip.
And who knows what Cindy told Caylee about her father, say in the car on the way home. Maybe Caylee got a little information, a lie or the truth, and then came home and repeated it to Casey, who was mad that this information had been passed to Caylee.
Of course, there are hints of even more nefarious fatherhood within the family... if the truth came out on that night, of something of that nature, plenty of emotions would be raw and possibly violent.