That is fertilizer and would help the tree...what might be happening is they have chewed on the bark, and have "girddled" it somewhat.
I can't imagine there would be enough rabbits to do that.
I know that once they find a food source they like it is really hard to get rid of them. I live across the street from an empty field and get wild bunnies all the time. We've built birdnet frames around our plantings to keep the 'pesky wabbits" and birds away from our plantings. Definitely look around the bottom of the trunk for girddling!