How did Ryder crater end up in this shape?

Ryder crater is oblong (13 x 17 km), with a distinctive slump on its eastern side, and is located in the South Pole–Aitken (SPA) Basin, which is the largest (roughly 2,500 km in diameter) and possibly oldest (estimated age of 4.3 billion years) basin on the Moon.

Because of its odd shape, some question whether Ryder is two craters or one. This distinctive landform could have formed when an impactor struck the surface at a grazing angle (<15° from the horizon), or when an asteroid split in two just before impact. However, there is another factor at play here: Ryder crater formed on a steep ridge, and this kind of uneven terrain is often responsible for asymmetric craters.

Learn more: http://lroc.sese.asu.edu/posts/503