Dellavedova's technical explained:
"We felt that he made contact with his head and shoulders into the knee area of Horford, so we ruled that a liveball physical taunt technical foul," referee Ken Mauer said to a pool reporter."
Basically calling it a deliberate move.
The whole 'above the shoulder' ruling is stupid in this case. If Horford gets a flagrant 2 for a deliberate elbow above the shoulders, and Del gets a technical for a deliberate move at the knees...