Terrain Assets from UNITY

Due to lack of time, I used some terrain assets which Annabeth recommended to download from the unity website. If I was to attempt to create my own tree, it would have took me a long time due to the style in which I wanted them. So i used one of the trees that came with the assets package. Also due to something going wrong with one of my own mesh's from maya which was grass I used one of the plants also from the package. The problem was, when i walked over the grass, it randomly shot me in the air. This did it on most of my groups of grass so I removed them because of this.