I’ve had this happen too and didn’t understand it either. The closed loop always had fur in it so I think the critter was either leg or tail caught and the snare continued to close after the critter pulled out. Kind of a whiplash effect.

I haven’t tried this but I was thinking of putting a deer stop a couple inches from the end. A tail or leg caught critter would pull out but the snares wouldn’t self-close in the pack before being set. Just thinking out loud, I guess

I've had this happen numerous times when using wood supports close to the dam on castor mounds. The beaver grab the supports to put in the dam and pull the snare backwards until it tightens up and pulls loose. The support is usually found in the dam. Hope this helps!!

After reading a few more replies, I agree with SniperB that 2/3rds of the snart needs to be out of the water, myself, 2" of it under water. Those beaver, in a 10" loop, can get entirely through a 2/3rds below the water loop. But...2" of it below causes a more advanced...farther foreward catch nearer to the head as that snare has to close as the head goes through and the body can not. 9" loop, when I do it. 2" submerged.
I dont do it much.