Certified GKH Gardening Expert
This is most likely caused by over or under-watering. Either scenario would cause the symptoms you described. What are your watering practices? You should water your tomato seedlings enough so that you don’t let soil dry out or seedlings wilt. The soil should be kept lightly moist, not damp or soggy.
On a more serious note, however, it could also be "damping off" disease, which thrives in wet conditions (caused by over-watering). More information on damping off” can be found at the following link: