Here is the status of the Lander as I understand it – Philae bounced 3 times (the first bounce took it up 1 km) and wound up on its side against a cliff. It needs to move, or it drain its batteries by the weekend. So, ESA says they will try this : “We will deploy the MUPUS penetrator for 2/3 of the max. length and then insert it.” The idea (I believe) is to get the craft up on its feet and get it more solar power.
I get the feeling that Philae was just bouncing along (and the comet was rotating under it) until it came up against something vertical that blocked its path. The picture (see below) is pretty spectacular – I had to look at several times to realize it was taken pointing up, not horizontally.