I was a mass photobomber in the 90's
Did you consider that people would be annoyed by it?
There are always people who are easy to anger.
It can a funny prank with friends, but constantly photobombing other people can be really annoying. You could be ruining other people's photos of moments that might've meant a lot to them!
It's harmless, it's not like I'm actually ruining their picture.
What I'm trying to say is you actually could be ruining their pictures!  That's why I think it's good that you don't seem to be engaging in it as much as you did in the 90's, and why I'm suggesting you don't do it to strangers anymore.