There's a whole bunch of back and fourth on RCG where Rob Lebebvre (Arducopter) mentions chatting with Lorenz (PX4) and that 3DR was moving to the PX4 flight stack due to the BSD licence that allows for it to be integrated into proprietary code.
Arducopter doesn't need the PX4 stack in reality as Rob confirms here:
RC Groups - View Single Post - Pixhawk from 3D Robotics and PX4
RC Groups - View Single Post - Pixhawk from 3D Robotics and PX4
RC Groups - View Single Post - Pixhawk from 3D Robotics and PX4
RC Groups - View Single Post - Pixhawk from 3D Robotics and PX4
Rob here says that development money is going "into other community projects" maybe PX4, maybe Dronecode, but basically just saying money that was directed towards Arducopter is going to other development.
RC Groups - View Single Post - How to support Pixhawk and ArduCopter developers? Ensuring a future for APM.
Rob was even questioning why Arducopter should even bother merging in Arducopter Solo in the last week! "Most of it shouldn't even be merged because it muddies the water"
RC Groups - View Single Post - Official 3DR Solo ***Owner's Thread***
Does that clear matters up? So what's being said is that 3DR are suspected of moving to Qualcomm's Snapdragon Flight and it's being said 3DR are funding Lorenz's team to build upon PX4 code (although even this is questioned too, but basically it is being said funding is going elsewhere than Arducopter) for code that possibly doesn't have to be shared.
I should add I could add at least a half dozen more alarming examples to those above but there's enough there to show there's already rumour there that I'd fairly summarised.
I'm not concerned about this rumour at this time as 3DR have said Solo is and will continue to be supported and will get regular updates. As Arducopter are now available on a paid consultancy basis 3DR can pay for development that way.
And the final point in all this, this post like everything above is rumour and should be treated a such.