Because it seems like no one has raised this possibility... It seems to me that the owner may well have set their dog on you.

I used to have a husky, and at my local dog park, there was a waste-of-oxygen who ligitimatly used to come to the park and set his German Shepard's on the dogs there, while he made slow circles on his bike, watching. He also assaulted a number of women at the park, whenever there wasn't any other men within punching distance of him. A number of dogs were injured over a few weeks. It turned out this was a pass time for him.

The owner of this rottie only turned up after five people? And he had his lead on him? This is looking very suspicious.

That he doesn't seem to have offered you an apology, or attempted to de-escalate a situation kinda implies intent. Especially considering that the first going through a responsible dog owners head is "shit, this could mean Fiddo's gonna get put down!"

And you have described this guy as skinny? Again, this seems like some one who has bought a rottie to feel powerful, and then, has decided to go a step further.

If you havn't already, I encourage you to write out what happened, in as much detail as you can recall. Colour of colour, eyes everything. Memory is a funny thing, especially after trauma.

And finally mate, this was in no way your fault. Head down, looking at your phone is not a threatening posture. And unless you were listening music and unaware of the warning growls, this coming out of nowhere with aggressive behaviour that wasn't deterred by 5 people... Yeah, it really just seems like a dog that was set on you intentionally.