I didn't even look at the whole log before asking myself if you were a shapeshifter. I don't know what causes it but it's essentially just like landing a lucky blow at any time. My guess is it's some weird code based on the supposed "hidden luck stat" that allows you a parting blow like attack when fleeing, in this case you were entwined and couldn't flee but the attack still went through. Also, this has been like this as long as I can remember, ie back in early 2000's. As to why you did a cause critical, I'd guess that's something to do with your character not technically wielding any weapons and it's a random damage noun. As a disclaimer, I'm not a coder but I have seen this a great deal throughout the years.