Mechanical cause of loss of consciousness
Mechanical force can compress neurons and cause action potentials as you probably experienced in the form of hitting the funny bone. Strong enough acceleration of the brain tissue may be causing massive excitation of neurons as indicated by animal EEG study. It suggests that the loss of consciousness is due to generalized epileptic seizure. However, it is not fully agreed upon: quoting wikipedia entry on Concussion:


  It is not known whether the brain in concussion is structurally
  damaged or whether there is mainly a loss of function with only
  physiological changes.[14][needs update] Cellular damage has
  reportedly been found in concussed brains, but it may have been due to
  artifacts from the studies.[15] It is now thought that structural and
  neuropsychiatric factors may both be responsible for the effects of
  concussion.[16]


And following quote is from this


  The brief loss of consciousness that characterizes concussion appears
  to be the result of rotational forces exerted at the junction of the
  upper midbrain and thalamus that cause transient disruption of the
  functioning of the reticular neurons that maintain alertness.
  (Allan H. Ropper, M.D., and Kenneth C. Gorson, M.D.; N Engl J Med 2007; 356:166-172 January 11, 2007)