Try slicing the stalk down the center lengthwise and look for signs of stem borers (hollowed out stem, larva).

Here is a link that might be useful: The longhorned borer Dectes texanus (Coleoptera: Cerambycidae)

I had that happen to me last year. It was a variety of some tree borer. the bug was bettle like but elongated and skinny, steel gray in colour and could cut through a stem super quickly as clean as a knife. Luckily I scouted the bloody thing (only one) and disposed of it. Have had no repeat occurrence.
Cheers Kim