The answer to 'why is my irrigation system erosion' is that your irrigation system is causing erosion in your landscape. This can happen when the water flow from the sprinklers or drip lines is too forceful, creating channels or gullies in the soil. Erosion can also occur if the sprinkler heads are not properly adjusted or if the system is not delivering water evenly across the lawn and garden.
Some common signs of irrigation system erosion include visible channels or ruts in the soil, exposed roots, and pooling or puddling of water in certain areas. To fix this issue, you'll need to adjust the water pressure, relocate or adjust the sprinkler heads, and potentially add soil or mulch to fill in the eroded areas.