Here on Vilya it's been good grim region. There were 6-8 on each side there the other night. I learned a lot. Like never 1v1 a creep that has more than 2 rows of buffs -.-. I can't afford those store-bought buffs. I have food and whatever buffs I get from how the map is. And that's it. /sigh...hard times. But I digress...
I had a 1v1 right outside of GV once. I tried to move down the hill a bit for 2 reasons:
1. the reaver picked up an add and I didn't want that to effect the outcome, and
2. I didn't want a freep to come down the stairs and blow up the reaver thinking something similar to what was posted in the OP.
Let's clear up something though. All of the moors is public. There are less traveled portions of the map, but they are still public.
Regarding the OP-er's warranted confusion regarding why the freep would have been running for cover: I think if there might have been some interference by another creep just passed the bushes and rock then that would have freed the freep from his obligation to stay and fight it out. Anyway, without talking to the guy and getting the facts we'll never know.