Like all chemical reactions, the unimolecular decomposition of ions is subject to thermodynamic versus kinetic reaction control:  the kinetic product forms faster, whereas the thermodynamic product is more stable. In the decomposition of  ABCD, the reaction to form  AD is thermodynamically favored and the reaction to form  ABis kinetically favored. This is because the AD reaction has favorable enthalpy and the AB has favorable entropy.
In the reaction depicted schematically in the figure, the rearrangement reaction forms a double bond B=C and a new single bond A-D, which offsets the cleavage of the A-B and C-D bonds. The formation of AB requires bond cleavage without the offsetting bond formation. However, the steric effect makes it more difficult for the molecule to achieve the rearrangement transition state and form AD. The activated complex with strict steric requirements is referred to as a "tight complex" whereas the transition state without such requirements is called a "loose complex".