Unless they have a unique item of gear from hell (which I bet they didn't).

Vokers sometimes set the place on fire because, once burning, someone can't camo or hide there.

More likely is that the voker spotted you before you camoed, or someone else did so he knew you were in the area, and then he proceeded to fireball you out.

I also tend to fireball in spots where you are more likely to have someone hiding. For example, my aps quite often iceball at the large crossroads on the eastern road and also the spot on the eastern road where going north will take you into the grove.

If I've seen a ranger on eastern, and he disappears, I'm likely to take a step into each of the wilderness areas and fireball or iceball there.

As a ranger, being unseen won't ALWAYS be enough. You also have to avoid being predictable. If you run to mount kiadana and hide when someone is chasing you across the ocean, don't hide at the first spot possible and stay there. Creep away so you can tell if he's fireballing people out, and better still, creep into a room with a bunch of storm giants so that fireballing you results in him pissing off a lot of giants. That sort of thing.

The other possibility is that you haven't perfected camouflage. If that's the case, you THINK you've camouflaged yourself but actually you haven't. (At least, that's how I seem to recall it works, but I always perfect it before I enter pk, just like I always perfect hide and sneak before I enter pk.)