The most reasonable rumor Ive heard on the radio (from an unnamed source within Arrowhead) is that Clark Hunt went to Dorsey and offered the same terms (in years) as Reid, and Dorsey turned it down and said he wanted to finish out his contract which ends in 2017. Hunt took this to mean he was hedging his bets -rumors of Dorsey reuniting with GB have always been there- and Clark basically told him that it would be best for both sides for them to part ways...essentially firing Dorsey, but Dorsey in this scenario sounds like he was forcing his hand.
Add that to the Maclin debacle, multiple years of cap hell, Berry not going to training camp because he was mad about negotiations...Dorsey is a solid talent evaluator but seems like there was some kind of rift brewing between players, coach, and GM