I had planted a grow-bag full of coriander seeds and they grew into plants with no problem. I planted some more seeds in another larger, wider pot, and after germination, a small clump of seedlings in a 2 sq cm area were suddenly flat on the ground one day. I figured a bird must've stomped it. Next day another clump of seedlings were flat. And so on until everyday a few more were getting flattened. I've been watering them every morning and evening, with just enough water to moisten all the soil. They are getting a good amount of sunlight too. On examining the flattened seedlings, I noticed that the portion of the stem that just emerges from the soil, appeared to be a bit squeezed, as compared to the rest of the stem, so I assumed it could be due to there not being enough water and the soil drying up. But even pouring more water didn't help, as today few more of them are flat on the ground. It's happening only in this flower pot, and it's the same soil I've used in the other grow-bag.
Update Nov 2019: All the coriander plants eventually died. Now a year later, even garlic plants are dying off. They grew fine for a few months and now all are listing and withering off. The curry leaf plant in the middle seems ok. There are black ants that used to use the bottom area of the pot as a storage location for their eggs, and they seem to have created their nest in the mud in the pot. I wonder if the nest building of the ants is creating some air pockets and introducing fungal infection or something.