The simple answer:
When Windows resets the I/O device count it sorts them alphabetically and picks the first analog output as the default. This is a Windows preference, not preference set in our control panel.
Mac OS X does not have this issue, it is a unique issue to just Windows (7, 8.1, 10), maybe Microsoft will fix it in a future OS update.
The deep technical answer:
Audio I/O that does not get removed from a system, such as on-board PC motherboard sound card / audio interfaces or simple fixed I/O detachable devices like the AudioBox USB have a fixed number of inputs and outputs and setting the default out goes to main out L/R by default.
However for more complex multi-input / output audio devices like the Studio 192 / Studio 192 Mobile / (or even Quantum) where the number of I/O can change depending on sample rate selected, then Windows will reset the default output because it does not remember what it was originally before the I/O count change.
For example, upon first connecting the Studio 192 to your system, opening Sound Output Tab you can select Main L/R as default output. At 44.1 or 48KHz which are default sample rates in Windows, the number of inputs and outputs stay constant. Opening a session in your DAW and working at a different sample rate say 88.2, 96, 176.4 or 192 will cause the number of Inputs and Outputs to change. Even though this is an ASIO operation, the device firmware will change the I/O available to the system. WDM Audio will see change and do an I/O reset.
Once you leave your DAW or (if you've not plugged in your Studio 192 in a while) your system will rescan the inputs and outputs for the available support sample rate and will reset the output to Headphone Out 1/2.
