After being in corporate American for the last 15 years, I'll agree that often its because of mergers and such. But just as often it happens for no reasonable or apparent reason. Sometimes management just reads a new book or goes to a seminar and rethinks their org chart, usually with the same terrible results as the last reorg.
Oh how right you are! I have been in a multi-national corporation for 33+ years, and when some corporate executive gets his/her panties in a twist, or reads some new "cool" book, or hears a "wow" seminar, you can bet that it means jobs are lost at the peon level, either through "Resource Actions" (ie. layoffs), or jobs to India, Brazil, Argentina, etc.
Last year was the year of Lean, a manufacturing process. It translates very badly to Mainframe system support, but they did it anyway. Got rid of almost 30% of the folks through layoffs and transfers, bloody *BEFORE* the process was in place. *sigh* what a mess, but some corporate exec got a big bonus.