Continued military operations apply pressure on Obama
It works in Israel's favour to continue military operations as long as possible, that is well past the US Presidential Inauguration. Thus Obama will have to show his stance on the Israeli-Palestinian conflict.
It is unlikely that Obama will move against AIPAC (lobby org) and in particular after his statements last year affirming that Jerusalem should be the Israeli capital. However, Israel might have felt insecure about the incoming administration's policies and signals, thus going ahead with this military offensive.