This is not a new phenomenon, in fact I remember a similar thread about cars following those picking up passengers. I also remember reading about a gang who was doing this out of SDQ. If I remember correctly, they stopped one car on the road to rob them, they didn't even wait for them to get home, so it may not matter what kind of security is at a residence.
The possibility of someone in the neighborhood being involved isn't out of the question either. My in-laws never tell anyone when we are coming - another problem Dominicans have is their houses being robbed when it's known that family has come from the US. They know suitcases full of gifts and purses full of cash are there.
When we fly into SDQ two or three of my husband's brothers come out to get us in two vehicles. Some of the larger luggage goes into the back of a pickup truck, and one of the guys actually rides in the back with the luggage. The other car rides close behind it. It's not unheard of for luggage to be stolen at traffic lights [so they tell me]. They are very, very aware of everything around us.
Thankfully, we've never experienced anything like the unfortunate incident the OP related. Since her son-in-law is Dominican he is likely more aware of the dangers than foreigners would be - if it can happen to him it can happen to anyone. Thank God no one was seriously hurt, there could have been an awful ending to the story.
AE