Page 295 Westons book
Reasons for divestitures included changing strategies, reversing earlier mistakes, and learning. (This is not a comprehensive list*)
Example; Divestiture by Biocon of its enjymes units and statement by Kiran Majumdar on it.
Example: Divestiture due to regulation
Economic Times Dated 3 August 2007 page 32
Arcelor Mittal said it will sell its Maryland steel plant(formerly Bethlehem steel) to Emarck Inc. for around $2 billion. The reason for this divestiture: US regulators ordered Mittal Steel to sell off the plant as part of its takeover of Arcelor because the new company would have too much market power over tin-plated steel output, used to make cans for food, aerosol sprays and paint.
More details will be given by the company after it received regulatory approval for the sale. The deal is expected to close in the fall.