NAC N272 going into mute by itself

I have my NAC N272 connected to a NAP100 and inputs are digital input from a MAC Mini and also an optical input from my TV.  Very simple system.

For some reason i do not understand the NAC 272 goes into mute by itself rather frequently, and this started couple of days ago for no apparent reason.  It is more frequent when i am watching youtube clips on the TV, although it also happens when i am listening to normal music from the Mac Mini using the J.River Media Player.

Any ideas why this is happening, and how I can eradicate it?