In some cases, catastrophic fracture happen because of not one but a combination of factors.
1. In your case, using smaller boat rigging in much bigger boat, you will always run a risk of breaking your mast. srm is correct here. One factor is boat weight, second is the width, third is draft. There will be a limit your boat can take, -- perhaps it is 18 or 20 knts. If a big gust comes in, H16 leans and spills the wind, and in worst case gets dumped. In your case, this doesn't happen, or not to that extend. All the excessive force goes into bending stress in your mast.
2. Putting an extreme compression on your mast would not break the mast by itself, but could been a factor nevertheless.
couple of us had to pull hard to get it attached
This could have resulted with a 300lb of extra compression on the mast and perhaps reduce mast rotation.
3. Last factor would be mast rotation. I still think, the failure should not happen with the mast properly rotated. Perhaps, it got counter rotated, or it was not quite properly rotated on the tack. This may not have been a sole factor to cause a failure, but in combination with #1 and#2, it could have.
4. This is a long shot, but there could have been something wrong with the new (old) mast. Perhaps a small dent in the middle of the mast? Or perhaps, the mast collected salt water and was left like this for a decade or two. This, in theory, could have corroded the aluminum from inside. Again, a long shot, but just to rule it out, you can cut it open and inspect.