I wonder why "Comment est-ce que cela s'est passé?" was not accepted. I remember "Where did that happen?" was "Où est-ce que cela s'est passé?" or "Où cela s'est-il passé?" or even "Cela s'est passé où?"

I was of the understanding that "se passe" meant "to happen"?

It does appear in the drop down hints on this question, but I was marked incorrect for using it.

Please could a moderator explain why it is not accepted in this instance?

Merci Beaucoup en avance :]

Is "Comment est-ce qu'il s'est passé?" also acceptable? Merci.

comment est-ce devenu ? - devenir : to happen - but DL does not agree.