A variety of factors could have led to the crash or "hard landing" of one of the helicopters that helped carry out the raid that killed Osama Bin Laden yesterday. First off, we've heard reports that the choppers used in the raid, which apparently involved a total of about 40 operators, were a mix of MH-60 Black Hawks and MH-47 Chinooks. Both of which are flown by the Army's 160th Special Operations Aviation Regiment. The AFP picture above shows what appears to be the tail section of an MH-60 draped over a wall at Bin Laden's compound. There are still no reports of U.S. casualties during the raid, which is impressive given the image above.
A mix of Black Hawks to carry in the initial assault teams (roughly 24 SEALs) and the larger Chinooks to bring in supporting troops as well as carry away any prisoners makes sense. Keep in mind that the crew and passengers of the downed chopper had to pile onto another bird (probably a Chinook) to make their exit.
In any case, a chopper on final approach to a raid insertion could have been forced down by small arms fire (a lucky shot to the gearbox), brownout conditions where dust kicked up from the rotor wash interfere with the pilot's ability to see, or it could have clipped some of the nearby power lines you can see in these pictures of the site (although there don't appear to be any downed lines) or a combination of all factors. Or maybe, it really was mechanical failure. Keep in mind the raid was conducted around 1:00 in the morning so they were using night vision goggles. This goes to show just how tough missions like this are, even for the pilots of the 160th, whose skills are legendary in the helo community. Those pilots have also had weeks, at the minimum, to practice the mission at a site that was built to reflect the compound and its environs. So they probably knew where any potential obstacles were and how to avoid them.