Grignard formation and vicinal dihalides
Taken from the Virtual Textbook of Organic Chemistry, which is maintained by William Reusch.


  If two halogen atoms are present in a given compound, reactions with
  reducing metals may take different paths depending on how close the
  carbon-halogen bonds are to each other. If they are separated by four
  or more carbons, as in the first example below, a bis-organometallic
  compound may be formed. However, if the halogens are bonded to
  adjacent (vicinal) carbons, an elimination takes place with formation
  of a double bond.


He does however not provide any explanation for this behavior. It could be that this result is largely empirically based, about some aspects of the Gringard reaction mechanism there is not much known about (or consensus among chemists). I looked at Clayden et al. and Vollhardt &amp; Shore and neither give a satisfying answer. If I find any in an other perhaps more advanced textbook, I'll edit this answer.