What are the current accepted theories of the formation of the Uranian moon Miranda?
As Miranda is a rather small object (only 235km diameter) it may simply have not enough mass to make it matter settle into hydrostatic equilibrium under the influence of its gravity, thus it basically retains the irregular shape most asteroids have. This nice arxiv paper discusses the mass limits necessary for a stellar body to settle into a nice round shape:

http://arxiv.org/ftp/arxiv/papers/1004/1004.1091.pdf