Response to outside pressures, I guess
Consider the following:
Samsung working on Tizen - possible loss of Samsung Android business in long term.
BlackBerry go to Foxconn for manufacturing - reduce their device cost
Google rapidly do deal with Samsung which may mean they will abandon Tizen, and flog the phone arm to another Chinese manufacturer so they are not competing with Samsung, LG etc., while also being able to undercut both WP8 and BB on device costs at the low end of the market. If we think of the Moto G as a proof of concept that they hawked around potential buyers, it might make sense. Motorola might lose money on them, but only until the new buyer took over manufacturing and started to make a profit.
They have had several billion back from the Motorola deal by selling set top boxes, taking the cash in the bank, and obtaining tax credits. In the end I suspect they paid only $3-4 billion for the patents, net.
So to me it looks like this deal is nothing to do with technology and everything to do with adjusting to the realities of the global phone business. It suggests that Google's commercial arm is the driver.