The clip you were using at the time was it a carabinier? Those are not reliable at all they keep opening all the time at the sligthest pressure, it happens a lot.
If it was a tuna clip then it was defective cause those are bullet proof.
The SS locking chain link is great, but sometimes the snap clips are safer. That´s the reasoen I always use a snap clip instead of a pig tail in my shooting line.
Dan warned me of tuna clips once a while back with his kayak story. I ignored and mostly forgot the warning because in my mind I had the same idea as you. Tuna clips are bulletproof unless they break. That incident must have been caused by a faulty tuna clip or some other thing etc. I also figured as Dan said, I would feel the slack of my board coming loose if it ever did. These false assumptions combined with all my gear already having tuna clips meant I kept using them. Bad idea.
After getting near shore fighting a strong current back in on a beach dive and reeling in a perfectly intact tuna clip without the board attached. I feel foolish for not heeding Dans warnings before.
I still have the tuna clip. Its in perfect working order. Very heavy duty with a lot of spring. I also remember ritualistically double checking the connection before getting into the water. After the fact I still cant imagine or reenact exactly how it came loose. Especially near the end of a 3+ hour beach dive after working fine all day. But it did. The GPS tracking adventure Dan and Hardline helped me go on is proof to me that tuna clips can do what seems impossible. Either that or sharks are becoming really dexterous and mischievous.
One possibility I imagined is that pulling the float in (to get water, put fish on the board, store flashlight etc) repeatedly might cause a strange tangle that allowed it to come loose. I dont know. 
All my connections from now on will either be one of Dans locking carabiners or semi-permanent shackles like Dan posted (Pliers to get them on/off). Ive been using a few locking carabiners on my guns for quite a while now and they are great. To this day they can be tightly secured and loosened by hand. Unlike shackles and quick links the threads stay perfect in the salt.