Normally they're quite predictable, it's one of the things I dislike about that particular hazard.
But yes, when someone runs past and makes them pre-buff for a group that's actually threatened by them, bad stuff happens.
Still, as others have said above, there's ways past them, but that road is not supposed to be a leisurely walk. It's in the shadow of Castle Ravenloft and there are, indeed, worse enemies lying in wait to ambush people on the road all around that area.
I don't think it's a great thing that you had to go back after NCE as a low level, and run into that "level barrier" -- but we've all had our harsh lessons and embarrassing instakills before. This one, at least, you have some measure of control over, with all the options you do have to get past them even as a low level, and you'll probably never have this happen to you again.