Why does this keep happening?

Additionally to already mentioned solutions I may add that you might have problem with soil. There might be some fungus that causes stem rot. Did you change the soil in all 4 plants or have you used the same source? It happened to me too with my first plant I was watering it too much. Any way good luck

Can't see anything wrong other than a bit of a lean, which isn't going to kill it. Just add some soil aroung the stem to help it stay up straight, but it really isn't an issue. If you have a fan blowing directly at it, that would be the cause. If so, adjust your fan so that it is not blowing the bejeezus out of your baby plant, which (possibly) might feel as though it is living in a hurricane. Air movement in the grow space is more than sufficient for plants this size, they don't need constant air directly on them to be healthy. If your issue is something other than the slight lean, you need to be more specific in your questions.

Don't find a diary for this - but if you keep getting stem rot (I think that's the issue), you're watering too close to her and probably watering too much. Water the soil AROUND the seedling and let it dry out completely before watering again... For this little one, I might try mounding up some soil around the stem ... some roots might start to form ... but just don't overwater! If you water AROUND, the soil right next to the stem will become MOIST but not wet.... and that's ideal... you want those roots that are forming to fan out looking for water... Good luck...