If a device generates clicks and pops at start and end of audio file playback, even when those audio files are completely zeroed out and dc-offset corrected, what could've caused them?
If your question relates to playback from a computer system, some audio drivers and codecs often have buffering schemes to stream audio to and from the hardware. I have found that some such drivers/codecs don't empty the queue of samples buffered up, even when the previous file is closed. This can account for a click at the start of playback. If you are getting a click at the beginning and the end it is most likely a driver issue.
Clicking in an audio stream is typically indicative of some kind of discontinuity in the time-domain waveform. I'm not sure what you mean by "zeroed out and dc-offset corrected," but these sort of artifacts can be created via unsophisticated audio editing methods. If you're suggesting, though, that an all-zero (i.e. pure silence) audio file will cause this behavior, then there's probably some little implementation detail that's causing it. Examples:
Audible effects caused by turn-on/turn-off of the output audio amplifier. This is relatively common.
Significant differences between the initial/final sample values and the output level forced to the DAC when "idle" (I can't think of a sane reason why the designer wouldn't make the DAC output zero when not in use, but it's possible to do something else).