sluggs, snails, caterpillars, earwigs are most likely suspects.
HERE IS WHAT YOU CAN DO FIRST:
Go out around mid night, with a good flah light and find out. Some caterpillarS and similar can cameflage. Look closely.
The chances are you will find them. When you do, squishem(maybe need a plastic glove. lol).
If that was the case, continue the drill untill you've got theM all.

Similar thing has been happening to my broccolies too. I found out that they are cabbage worms. They also damaged my cabbages.Surprise!
I also do have some sluggs and snails, but not very serious YET.
So we have to fight many battles. The war is never ending.

Before doing indiscriminate squishing (some things NEED to be squished, some don't) please try to find out WHAT you're squishing. The owner of a nursery here in town is beside herself because even if she prominently posts pictures of swallowtail butterfly caterpillars saying "PLEASE DON'T SQUISH" folks come in all proud telling her that they've killed tomato worms. Which they don't look like. Google will help with pictures, and so will folks here.

I've never had the problem you're describing, but is it at all possible that you've got something like grubs or slugs that somehow got into your boxes that are eating the roots, which leaves the poor tops of the plants to blow away in the breeze?

Don't give up! You'll get TONS of good information and guidance here.