Since you are just asking for guesses, I am going to guess overwatering. Overwatering is next to impossible to avoid when seed starting or growing on with a fine grained mix which almost everyone uses.

I recall one year I started a few flats in pure vermiculite. Many had said they used vermiculite with great success, but every plant in the vermiculite was scrawny and had seriously underdeveloped roots.

The two are tied together, of course. If the roots can't establish, the top won't do well either. Over watering leads to root death. Vermiculite, by virtue of being a water sponge and fine particled basically suffocates roots.

When most start seeds shallow containers are used. Fine grained mixes will always hold a perched water table that doesn't drain from gravity. In many years this won't be a problem because mix volume is so small the plant and evaporation dry out the container quickly and oxygen returns to the roots.

Let there be a year where lighting or weather conditions don't favor rapid plant use of water or evaporation of the water and root growth is killed from the low oxygen conditions and top growth likewise suffers.

It's the #1 reason why I don't use fine grained mixes for seed starting or any other purpose. They hold too much water and not enough oxygen for proper root growth unless conditions are favorable for rapid drying of the limited mix volume.

Pot up...sunlight too.