Not a geneticist either, but this is something the book "The Gene" by Mukherjee can explain.

Quoting from the book (chapter "Regulation, Replication, Recombination"): "A gene, in short, possessed not just information to encode a protein, but also information about when and where to make that protein" ... "Proteins act as regulatory sensors, or master switches, in this process — turning on and turning off genes, or even combinations of genes, in a coordinated manner."

So you have a feedback loop, whereby a gene encodes a message to build a protein that regulates a gene. To get back to your original question, a mutation might encode a different protein which might react to some environmental changes to produce a change in behavior.

Not medical or genetic expert but I would guess that, considering it seemed to be triggered by happiness, it's a sort of overload on some system in the brain.

The child goes happy -> something overloads -> something shuts down -> recovery to normal