This is not normal. Generally this means that the drainfield is not absorbing water and that since the water cannot drain downward, it is coming to the surface as the path of least resistence. It is likely that if this continues, water and black sludge will actually pond in the yard. 

If you have not, you should have your septic tank pumped along with the drainfield. Find your distribution box so that you can monitor the field in the future. A treatment with Septic-Scrub can help restore your drainfield to better operation. 

Please call if you have any questions.