Asteroids don't have enough enough mass for gravity to pull them into a spherical shape. The smaller ones, in particular, can be very loosely bound. Here the Hubble Space telesocpe catches one in the process of breaking up with the pieces slowly drifting away.. It is unlikely to be the result of a collsion - that would have been much more dramatic. Rather it is likely that a spin was induced by the pression of sunlight and that was enough for a breakup.