Just awful and obvious jealousy that they have to ruin someones hard time and effort spent.
I had a simular problem with a neighbour from around my area. I used to wash my 350z every Saturday and spend a good few hours detailing it. For me it was more a hobby and kept me entertained at weekends. After the first few times I never had any problems, but then all of a sudden I'd wake up Sunday morning and eggs would have been thrown at it. In the end I set a night vision camera up and found out it was a 40 year old guy from across the street doing the damage.