[OK, for starters, I just want to say that I don't actually believe this, even though as far as I know, I made it up myself based on all available evidence as a means of explaining the disarray and hopelessness of the present situation. But at this point, it seems as plausible as anything else.]

The team isn't losing by chance.  Don Nelson is doing this on purpose.  He's sabotaging the team, playing vets who aren't winning at the expense of the morale of the team.  He's being haphazard with the young guys to ruin their development and confidence. He's making sure that rotations never gel, that players get frustrated and quit.  He is, driving this team back into the gutter.

Why? To get back at Cohan for the way Nelson felt he was treated after the first go around and steal some of his money in the process.

It hasn't been easy.

He saw the opening when Mullin convinced him to come back and convinced Cohan to patch things up.  Nellie then had to win enough to get an extension, one that would pay him well long beyond the time he ever intended to stay here.   He had to make it look like things were heading in the right direction.  The Dunleavy/Murphy swap, the resurection of Jax, the building of Barnes. Then, by means of getting on Rowell's good side, he planted the seeds of sabotage in contracts ("Hey, Bobby, don't keep Davis, he's a bum.  Turn that money around and give it to Jax, who is certain to be the leader well into his mid 30s"; "Sure, I know that Crawford's deal is longer than Harrington's, but I really think this is going to turn us around").  

Meanwhile, on the court, Nellie institutes a rotation designed to destroy morale and prevent any player development.  Cohan will have enough and can Nellie, who will sue to collect the extension money while Cohan's team spirals down the drain, left a mess for years to come.