Wondering if the line was wrapped improperly around the POW. If you change the direction of the wraps after the half wrap it can lock off. I've seen this happen, even from an experienced groundie.
No damage and had their been any hazards in the DZ we absolutely would not have taken that big of a top. And yes it seemed to run appropriately and 100% failed at the POWHonestly, before I even clicked on the thumbnail or read the description, I thought that piece looked way too big for that rope. (9/16 is pretty sturdy, but that was a big top.)
I'm guessing you cut it there because that was the limit of the bucket. I'm not knocking you because I've been there. I probably would have used a bigger rope, or sometimes I'll actually bring up an extending power saw and cut the top out with that. Gains you an extra 12 or 13 feet, and with a top that diameter saves a ton of weight on the rope. Obviously, climbing out to get the top is also an option, especially in a nice live tree like that. (Assuming no defects I'm not aware of).
As to why it broke where it did I'd say that was the highest stress point and there was simply too much weight. Weak points are bend points so either the knot, the bend over the block, or the wraps on the porty. Looked like a huge amount of stretch before it broke, but also looked like the groundman let it run appropriately- so guessing it wasn't locked off. Just too big.
Glad it looked like no damage.
Breaking strength of that rope is something like 12-13k lbs I believe. Needless to say the stable braid is in the mail!No damage and had their been any hazards in the DZ we absolutely would not have taken that big of a top. And yes it seemed to run appropriately and 100% failed at the POW