It is highly unlikely. Unless, of course, the shackles were old or weak. Maybe the buckles were weak or some such otherwise. But, if the shackles were of good quality, new, and the buckles were solid, you're more likely to yank the rope or otherwise off if they're not tied properly or the rope was weak. In any case, as long as everything is solid and new and/or well tied, I personally don't see it happening.