It sounds like this was a gang initiation stunt. It was probably teenage boys doing what they had to do in order to gain acceptance and a sense of belonging, and protection. Their lives are really at risk, and they need other gang members to survive, is their belief. So they do what they are told. Once in a while they accidentally kill somebody and become hardened criminals. I was a counselor for young men who did this kind of thing.
Yes you have PTSD for good reason. And you are lucky to be alive. You were wise not to open the door. But the chances of something like this happening to you again randomly are pretty slim. And I doubt they were targeting you in particular, unless you accidentally "dissed" one somehow without knowing it. The police should be able to provide you with validation of this information, because there are probably a lot of other similar incidents. They know what is going on. Too bad they didn't explain it to you.