Bail that pit out with a wet shop vac, bucket or coffee can, dig out any solids (mud) sitting in the pit and see what you have going on.
It's pretty common with drains like that where the actual drain outlet is on the bottom of the pipe (unseen in photo). When solids fill the pit to the point they reach the drain outlet water can no longer get out and that is how you know it's time to clean the pit. If things drained straight into the end of the pipe, the solids would enter the pipe causing worse problems.