I agree it should not fail like that but that's quite a long seat post you've got in it and if you tend to sit back on the seat that's alot of pressure on the frame right where it's cracked.... looks like the paint is also cracking on the down tube underneath the top tube as well.. Just looks like the whole frame is flexing in that area and it's slowly weakening the frame..

How far down inside the frame is the seat post ?

When you replace it maybe go for the next frame size up so you don't need such a massively long seat post..

2p