Appears to be environmental glitch.
What is the potting mix?
Do the containers have drain holes?
How moist/wet is the growing media?

I'm using a regular "universal" soil (it's really poorly described on the declaration), nothing added. Seemed to work last year well so I didn't think any chance is necessary this year either.

No drain holes as I tried to water just enough to wet the soil and not drown it. This might be a problem if I wasn't careful enough and did over water some of them.

As stated above, I try to wet the soil and then re-water when it gets almost completely dry.

Must have drain holes because you must add enough water to thoroughly moisten the growing media with 10% or so extra to run out.

Further, need lots more light.

Are you using fertilizer yet? If not, start with 1/4 strength.