Does she get enough exercise? All dogs, especially larger breeds such as this 'must have good daily walks' -two if possible. The dog could be bored & have energy whichs she needs to release, this may be why it is currently being focused negatively.
If she lives out in the garden & not in the house she may not be getting enough attention, or maybe she just needs more mental stimulation?
Otherwise something could be unsettling her, it depends how long you have had her & how long this behaviour has gone on for? Dogs need time, attention & understanding, they are intelligent creatures & there is always an underlying reason for such behaviour. By strengthening your bond with her the reasons for why she does this may become more obvious to you.