Alright, that makes sense now. Dead wood, no matter the species, is more brittle than green wood obviously. Those looked like long, large horizontal leaders, which are exerting a very large tension force on the top of the limb. You did a simple drop undercut, but not nearly enough to stop it from tearing apart, basically like a barber chair, and it didn't tear as much because it was dead and dry.
The solution is to either climb out farther and take smaller pieces, or to approach the cutting like an extreme head leaner. You can do a coos bay cut, where you cut in on one side only, then cut the other side until it pops. You can also do a shallow face notch, then bore in to form a small hinge, then cut up if needed, then cut the holding wood. You can also ream the drop cut as it starts to close, but since that requires reaming with the top of the bar, I've never done that.
Also, from what i can tell from the pictures, you did your top cut father out than your undercut. Doing that increases the likelihood of your saw getting snatched, and that's no good. Also, be very very careful with putting your lanyard around a huge horizontal leader you are cutting, if it tears it could crush you by splitting farther back and jamming you into the tree.