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.
That looks like one of a number of soil-borne fungal diseases that are collectively known as "damping off". There's no cure that I know of - it's best to avoid it by using sterilized soil-less mix in your pots. Having good air circulation while the seedlings are vulnerable is also a good idea, as is consistent watering.
Pot size can also be a factor. Your pot is way too large for the seedlings, in my opinion. Too much soil/soil-less mix holds too much water and promotes a damp, usually cold environment, which just happens to be another thing damping off likes. I use either 20cm x 12cm x 3cm trays or 48-cell or 72-cell trays. Once the plants fill the cells I transplant them to larger containers.
One thing to keep in mind in this situation is sowing in loose soil. If we look closely at the seedlings in the lower left quadrant of the posted image we can see that there is almost a centimetre of light tissue between the growing point and the soil surface. One way this can happen is if the soil is overly loose when we add seeds. The seeds germinate and start needing more water. As we top water, the loose soil starts to settle down and as it does so it goes to the bottom of the pot and around the lower roots, leaving the growing point stuck up in the air. Roots are not designed to have structural strength and so the plant flops over. Plants may recover but have suffered a setback.
So it comes down to how much we tamp down the soil when we sow, when and how we irrigate, how deeply we sow and how long germination takes. If germination takes a long time, by the time it happens perhaps the soil has settled. Sometimes ebb and flood irrigation, through the holes in the bottom of the pot, keeps more fluff in the surface medium. If they are up and away fast and then their pants fall down they are left feeling a bit awkward.
In this case it looks like the coriander seed was sown in regular garden soil, somewhat clayey, which can be lumpy and clumpy, settling irregularly, leaving fragile roots exposed.