Is this some old pagan Slavic (heathen) ritual, or just a stockpile for wood an wooden sticks for woodcutting purposes?
Sorry, no. Just simple prevention from falling.
If that is the reason, then there must be some history behind why people started doing it. I tend to think that Pieter is right, that it may have had pagan/Slavic roots. They must have believed there was a spirit or god or something in the rock that led them to believe it was a good thing for them to help prevent the rock from tipping over.
Every custom begins for some reason or another...they don't just happen!