Basically - he was told to clear up his mess first, and sell the players that were sitting in the reserves on big contracts.
He was asked several times to move some of the big wage players he wasn't using on - basically, to manage his squad like every other manager does.
He kept saying he would but then didn't. He kept asking fr more money from Lerner, which he got, but still didn't move players on. The final argument was because Lerner wouldn't let him spend a huge wad on Aiden McGeady until he'd moved some players on *first*
That's specifically what he walked over.
Incidentally, his media backers saying Short hasn't backed him despite him spunking 25m on just two home market players sums up everything about the bloke.
iirc there was a huge pool of 'back up' players on 50-60k