This has happened to me on quite a few occasions as well...
The best I can come up with is that the battle starts to lock up after a few turns pass with the boss having lots of debuffs/status ailments applied. I had this happen more often when I used Mordred, as her Outrage does just that.
It happened less often if I didn't use Outrage at all. If anyone else could test this, it would be great.![]()