The BBC's explanation for headlining the story with the Israeli response rather than the original Hamas attack was this:
'A spokesperson said: "The original story led with Israel's air strike on Gaza because this was a new development and the first time Israel had launched an airstrike of this kind since November. The article fully explained that the strike followed a Palestinian rocket attack on the previous day. As is usual, the story has since been updated with further developments and reaction." '
What is not explained by the BBC, is why the BBC didn't report the Hamas rocket attacks until after Israel retaliated. I can help them out; it's because the BBC is institutionally biased against Israel.