Dog Underground

Why do this dog keep putting all her chewing bones underground?

She keeps on doing this. Everytime i gave her some bones to chew to keep her busy when im not around, she tends to rather put these underground or hide and cover it with piles of grass???

She's a jackrussell. 1.5 yrs old. I got her from the dog pound 2 weeks ago after being ditch by her owner. She is a scaredy type of dog which gets nervous on everything... She is worried again now that someone might steal her bones???

Thanks

Sounds like she is protecting her food source.
Possibly she has had to compete for her share of food in the past, so now she buries it to protect it.