Traditions grow out of fact, oftentimes lost in the mists of Time.
So what could have been the inspiration for these costumes?
The Yeti from the Himalayas - the real ones, not the robotic ones as seen in 'Doctor W\ho' - is one possibility.
Or they could be the remnants of a cultural memory of alien visitations in the Bulgarian region.
I of course prefer the latter. Much more Toobworldly.