I think it has something to do with owning rather than leasing. I'm no expert when it comes to voodoo accounting. I remember the first 777 being scrapped and wondered why, did a little digging and found that on paper it wasn't worth a cent and they could make more by scrapping.
It's actually not so much voodoo accounting (nor lease/own - to an extent) for a part-out, it's more about the same supply/demand economic dynamics that affect every part of our industry.
Generally speaking, an airplane will be parted out when it's value can no longer be extracted by the owner. For an airline that owns the airplane, the part-out usually comes when the sum of future maintenance payments outweighs the revenue potential of the airplane (similar to a car). For a lessor/financier that owns the airplane, the part-out usually comes when the residual value (and hence, lease rates) has dropped to a point that can no longer recover the initial investment in the aircraft - this usually happens if there is an oversupply of aircraft in the market, or if technical obsolescence has dried up demand (think 737 classics vs. 737NGs). Other situations could be fuel prices climbing to levels that make the older-aircraft uneconomic, interest rates being low enough that new-aircraft financing is more attractive, new environmental/noise regulations, etc.
You enter a unique paradigm when individual shells can be parted out and their parts are worth more than the airplane itself. The same dynamics apply as above - something has driven demand for that type, or that specific aircraft, to uneconomic levels. For both airlines/lessors, these are usually "first off the line" or "last off the line" deliveries - these types are stripped for parts rather than being remarketed and sold/re-leased. The early manufacturing hiccups usually burden the performance of the plane (think re-work in the factory adds weight to the plane), making it difficult for the operator to justify the expense for little-to-no performance savings. With a large backlog of future deliveries, parts from the engine, landing gear, avionics, etc. can be easily stripped and sold as spares to carriers that need them and can now get more attractive rates with increased supply in the market. Same applies to last-off-the-line airplanes - their parts can be used toward either next-generation deliveries (where parts are shared), or toward the current-generation in-service fleet.
TL;DR - it's all in the economics, not really any different than selling the seats on the bus!