This is climbing gear, failed years ago, but the message applies to sailors. I have cracked toggles, cables, and shackles. These sit on a book shelf, across from my favorite chair, as visual reminders.
Fatigue. In this case it seems clear that the latch pin was fitted a bit too tight, and after some years, started a crack. since the strength of a carabiner is dependent upon gate integrity, this one was retired, certainly only a fraction of it's rated strength.
Open gate strength. This biner snapped during a fall when the gate was pressed open by a bump on the rocks; with the gate open biners loose 50-80% of their rated strength, depending on the design and how they are loaded. Fortunately there was a back-up and there was no serious consequence, just a clear lesson; the gate MUST be protected from even light contact.