Are you the builder?

If so, it might be (just a possibility among others) the result of overheating, or overly quick cooling when brazed.

Thin walled tubing cools very quickly (by steel standards) and rapid cooling can render many alloys more brittle than otherwise.

Of course, there might be other possibilities such as stress risers from a prior non-catastrophic collision, though I see no obvious evidence of an upset.

It's hard to draw a conclusion from a photo, and often the best clues lay elsewhere, like the fork, or toptube, neither of which were shown.