Steam VR tends to go back to its default “blue room” under a number of circumstances, e.g. if the lighthouse sensors get desynced, the headset loses connection etc. Since the game apparently still is running as it should, I would suspect something like that.
Try to keep an eye on the SteamVR status window, usually one of the device icons starts to flash in such a case and/or some message may be shown.
Also try to restart SteamVR if you did not try that already.