seems to me the damage caused by

1) forcefully screwing the waste onto the sewage drain pipe despite huge gap. in this case need to make sure the gap not too big, coz changing another new sink will suffer the same fate.

2) some kids step on the hole gap, unlikely scenario I think.

another possible explanation: it was compressed from the side eg. during car braking.

another 370 mystery...?

just braking alone ofcourse wouldnot cause that. but if ssecured withsome heavy objects like a few carton of tiles then the picture might be different. else some1 of 60 kg above . stepped along its long plane. such incidents might reveal some stress line along the part connecting the 2 bowls.

good case study for further experiment between 1.0 n 1.2 sink