It is a normal behavior for a bear to scratch rough material per itchiness and such. Bare in mind that bears inhabitant only certain ranges across North America, and you may need to research whether they live near your area. If you would like to find out what caused those scratches, start experimenting ideas. What are the shapes? Are they high up in the tree or low? Perhaps around two meters high, the typical height of a Grizzly bear? Is there anything up in the tree that a bear could have been attracted to?