Provided you have a genuine 5ft Salmon Poly leader at 24lb b.s and not a 5ft Trout Poly leader at 12lb b.s., there's no way on this gods earth that should've happened, I can only make an assumption you got a sub std item (for whatever reason!).
I've used the Airflo polys for Salmon more or less since they first came onto the market and never had a problem with breakage, right enough I did have a problem with the Rio 10ft Salmon Polys. once, but that was some time ago!.
Of course you are making the assumption you were caught up by the flee or cast material!, you could've been caught up on the poly itself somewhere, in which case?.
Pedro.