Here are the reasons why these teams want to dump the contracts:
Washington - they are $6 million over the luxury this year and way over for next year. They are not a team that can afford to be a repeat offender of going over the luxury tax. They don't want to trade Beal, Wall, or Porter. Mahinmi is the obvious casualty for an asset or two.
Houston - they have tried to move Anderson. Why? Because his contract hurts their chance to contend. They could stomach this year because they have Paul. They can't keep Anderson this summer and beyond because Capella, Paul and Ariza have to get retained. On top of that, they might need more talent to contend. They will use an asset to dump Anderson.
New Orleans - they traded a bit for Boogie and they want to keep him. To keep him and AD for that matter, they need to pay him AND put the team around AD. Asik is a massive roadblock to adding to Holiday, Boogie and AD. Asik has to go using an asset or NO loses a chance to build around an all NBA talent. NO is probably the least needy on this list, but they have a very tiny window with AD.
The situation isn't that rare. Yes, some teams can stomach a bad contract from time to time. However, these three teams can't if they want to get near their most important goals (avoiding repeater luxury, title contention, or keeping a roster together).