Why Does My Neighbor Act Friendly Then Complain to the City?
Humans are evolutionarily built to avoid open conflict with anyone close enough to retaliate. A neighbor smiles and files anyway because of: 1) proximity cost, he has to live beside you tomorrow and a paper trail never has to look you in the eye 2) the bystander shift, handing the fight to an authority means nobody has to own it 3) moral licensing, the friendly wave is what buys him permission to file. He is not two faced. He is efficient.
The pattern
The wave costs him nothing
The complaint costs him even less
Authority carries the conflict for him
You find out by mail
How it actually plays out
The real question is not why he filed. It is what he has already filed before.