well first too congrats to you
next as for why the arrow broke, , I doubt there will ever be a 100% for sure answer
but a few things that come to mind on me is, one, many folks shoot an arrow OVER and OVER and OVER, re use them after hard kits and several kills
and ALL that time, that arrow is becoming weaker from all the impacts, friction on shaft from being in and out of targets and so on
they are very much a ear and tear item, and many folks think that just cause they look good on outside that they are still 100% as good as new on inside, and that just isn;t true!
ALL things have a shelf life and a use life, and at some point, they should be retired and stopped being used!, lamination can also fail over time, heat exposures and heat cycles of hot, cold and doso on!
things get weak and with sudden stops, or being launched, they can find them weak spots and fail!
NEXT< is simply imperfection when being made, they can be there from day one, and again over time, eventually cause a failure!
another thought is maybe you hit an unseen twig or branch or? in its flight and again, that weak spot showed itself finally!
and the above goes for both carbon arrows and aluminum one's!
,
I know I was over kill maybe, by most folks standards, but , for me any arrow that hit anything HARD, got retired/scrapped,
any arrow that Killed an animal, also got retired, I sort of keep the kill ones as reminders and label the kill on a tag and attach it to them!
but over my yrs at ranges and having a gun shop and hearing stories or SEEING things go wrong, I tend to error on the side of safety over trying to get max use out of things! I will replace sooner than most, but I like my fingers and limbs a lot LOL(also have too many friends missing some to keep me thinking this way)
as I have seen strings fail on both vertical bows and X bows,(sen limbs fail too on both) and its NOT pretty when that happens, I have seen shafts fail on both as well, most times injury free, but close calls, and a few that ended with folks in ER's
SO< those are my guesses on why shaft broke as it did on you, just a guess, as that's all I think is possible here!