The only thing pest-wise that looks possible is some kind of cutworm. Think big fat caterpillar, nocturnal, hiding in the soil by day. Cutworms are usually slaying precious little seedlings, but I read that some cutworms do feed and cut higher up when plants are more mature. You could try lightly cultivating the soil between rows, to see if you turn up any culprits. Or you might try dusting the stems with DE anyway and see if that helps.
If the cause is purely physiological - brittleness due to ?? some growth/environmental conditions - then you should be able to snap a stem with your fingers.