Why would a DAC output an analog signal when losing lock?
Because the DAC gets confused about where to start reading music within the data stream.
Think of the data not as music, but packets, or envelopes. Each packet has more data than just music in it. In order to read a packet correctly you have to get the start of it right. If you start reading the packet in the middle, you misinterpret the entire packet.
For whatever reason, the DAC does not realize it is confused.
An easy way to try this is to feed a 2 channel DAC 5.1 from a HT source. It does not interpret any of the 5.1 data correctly so noise is all you get.
Best,
E