Gene,
Maximum stress is what breaks stuff. There is no time component. If the force divided by the area across which that force is distributed is higher than the maximum tensile stress of the material, even for a nanosecond...snap.
Force itself has a time component, of course. F=MA and all that rot.
Human error (not locking the gate) is pretty common. In addition, some locking biners, my big Omegas included, can get stuck in an 'almost locked' position sometimes. Usually a little hand filing will fix things.
Biner gates sometimes momentarily open when the biner, particularly its spine, is slammed against something. The biner stops suddenly, the gate keeps moving (and opens) due to inertia. If the biner's under a huge load, it might deform enough during that brief time such that the biner can no longer close. This is one reason why wire gates are inherently a tad bit safer in general: lighter gates = less gate inertia, and wires allow for more room for a stronger gate spring in the biner itself.
The twang of a slackline could also vibrate a biner fast enough to open the gate momentarily, I'd wager. Don't know.
I suppose that if you've got a mix of biners on your draws, its best to use the wire gates for the rope end...they're the most likely to be slammed hardest against the rock during a fall.