They will have become fixated on the cub because it's such a lovely machine.
Always hard to tell but the mistakes are
1. Aircraft trying to compete with another for banner towing
2. Bad lookout from everyone especially the guy in the Cessna, presumably because he was formating on the afforemention lovely cub.
3. Poor communication, either not all on the same channel or not reporting what was obvious
4. There was a change in height somewhere, the husky was flying very slowly presumably to formate on the cub but the Cessna clearly slowed and looks like it didn't manage to maintain altitude, the cub will have been back at about 60-65kts so the Cessna would not have been all that easy to handle while maintaining S+L and formation.
In isolation none of these would have caused an accident. I hate the "swiss cheese term" but the holes lined up good and proper.
Lucky people.
I still maintain that cubs are beautiful and is basically the hub of the problem here.