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!
Yes, the popular belief is that rocks are supported by gnomes but only at night. During daylight they support rocks with sticks. People add sticks if they want to give a hand.