Seed Rot

Sown grass seed has failed to germinate and the seed decays and rots. This is often termed Pre-emergence Damping Off, although for that definition the distinction is that germination has occurred, but the leaf has failed to emerge.
An increased likelihood of seed rot is due to poor seed bed preparation, using old grass seed, sowing seed at the wrong time of year, especially when it is wet and cold, and when surface drainage is poor or rainfall continuous. the soil retains a lot of moisture keeping the soil surface wet.

To reduce the chance of seed rot aim to produce a well-prepared seed bed, use viable seed, and sow when ground conditions are suitable and other environmental conditions are good.