Pathogens in soil can cause stem rot, I have seen young seedlings stems rot rapidly, perhaps it was something like that (which can be a by product of overwatering due to the disease thriving in wet conditions, air flow is also going to be a major element in this.....just my 2 cents.
Nice stuff COF, looks like you got a nice selection as usual!
I was going to go on to say that without getting into the why of why he's using 2g pots for seedlings. If you soak the soil in a fresh potting of a plant, seedling, whatever. That is what most people would do
when the container is the right size for the plant in the first place. Soak a big pot with a seedling and 99% of the water isn't going to get used at all and hopefully evaporate quickly enough to not turn into funky moss mud in the bottom of the pot. You pick up the pot, it still weighs 15lbs and you don't water it. Well that new sprouts got a root that's only a half inch to start with so if that top layer of soil dries only an inch or so down and bam, your plants dead.
To say seedlings are so easy to overwater simply means some corrective criticism was required. Seedlings are quite the opposite and damn near impossible to drown with typical over watering in a short amount of time. Growing ball funk in your soil cuz it's in a pot too large is another story too. The bottom line is the plants aren't dying directly from over watering.
If you must grow in such a large container from the start, don't soak the whole pot. The plants not gonna use it and you're just asking for problems. I start seedlings in 16 oz party cups and they get soaked to runoff everyday about the first two weeks, its not necessary but its easy and
I know I'm not going to overwater them that soon. Take that same plant in the big pot and water it about a half cup straight to the stem 1-2 times a day. The large quantity of soil will wick away and evaporate the water you are adding so you won't end up with the heavy bucket of mush. Anyways, I won't say anymore on the subject. I first grew in soil over 20 years ago so I have seen a sprout or 2 in my day. What's that saying, you can lead a horse to water.