This has happened to me several times, and I while I was running in debug mode I finally caught why...

If you start Picard normally, load some albums in, tag some stuff, etc. all the usual activities... and then you leave Picard open, and do nothing with it for a couple of days (not certain of the exact number yet, (like five or so).)

When you start using Picard again, it will sometimes suddenly crash. 

Well, this just happened to me again, and picard gasped this with it's last dying breath:

Traceback (most recent call last):
  File "ui/coverartbox.py", line 179, in set_data
  File "util/lrucache.py", line 67, in _getitem_
KeyError: -7494033226055751448During handling of the above exception, another exception occurred:Traceback (most recent call last):
  File "coverart/image.py", line 383, in data
  File "coverart/image.py", line 110, in data
FileNotFoundError: [Errno 2] No such file or directory: '/var/folders/g8/tg5c9z0j2zn69y6gdj8_51gw0000gn/T/picardla76bgbj.jpg'During handling of the above exception, another exception occurred:Traceback (most recent call last):
  File "ui/itemviews.py", line 207, in _view_update_selection
  File "ui/itemviews.py", line 299, in _update_selection
  File "ui/mainwindow.py", line 1462, in update_selection
  File "ui/coverartbox.py", line 373, in set_item
  File "ui/coverartbox.py", line 387, in update_metadata
  File "ui/coverartbox.py", line 252, in set_metadata
  File "ui/coverartbox.py", line 183, in set_data
  File "coverart/image.py", line 385, in data
picard.coverart.image.CoverArtImageIOError: [Errno 2] No such file or directory: '/var/folders/g8/tg5c9z0j2zn69y6gdj8_51gw0000gn/T/picardla76bgbj.jpg'
Abort trap: 6{}

So, it seems that OSX has nuked the temporary files in some kind of periodic housekeeping of old temp files.