As someone who spends a large part of his time at work at heights, dealing with the exact same hardware, and in some cases systems that are essentially zip-lines for "other stuff besides people," this doesn't seem to be a mechanical failure of the zip-line in any way. A zip-line is essentially a free-running Tyrolean.
I won't rule out that there could be a mechanical issue with the individual hardware, but there is very little that can go wrong with the systems. Most Zip-lines use either a Petzl Tandem Trolley, or something similar as a means to suspend the rider. They are very simple and don't "just break." It is very obvious when they reach the end of their lifespan, and unless the operator had zero procedures for hardware inspection in place, it would have been caught. I have a hard time believing that it was a failure of the gear.
Barring an improbable mechanical failure, my best guess would be that the woman stopped herself, either intentionally or unintentionally. Most zip-line operators provide a braking mechanism of some form. Quite often, it's a glove with a thick piece of leather sewn onto it, and the instruction given is to squeeze the zip-line when you want to slow down. Zip-lines don't always exist between radically different elevations, and commonly the momentum of the rider simply carries them to the end once they've surpassed the halfway point. However, because zip-lines are rarely taut throughout long distance runs, a significant portion of the suspension cable actually runs "uphill" after the halfway mark. If the rider does not have enough momentum once they've run through the belly of the cable, it's entirely possible to completely stop before reaching the end of the line. If the amount of belly in the cable is significant, they could even travers backwards to the lowest point on the cable. If the "pusher" and the "catcher" are not in radio contact and operating under the policy that there will only be one person on the line at a time, it's quite possible that the woman stopped herself, her husband was already on the line, and thus a collision.
It's a tragedy, especially considering that it happened on their honeymoon. It should not have happened. While the woman's own actions may have been the cause of the accident, she's most certainly not to blame. Even without any fault of the system, it seems as though the operating procedures in place led to the circumstances where this accident could occur.
My opinion is from a work-at-height background, and deemed a "Competent Person" at work. It's an OSHA definition. Essentially it means I have the expertise to identify risk, make recommendations, and have the authority to shut work down if I deem it appropriate. I write risk assessments, develop rescue plans, etc., for situations just like this. And provided that all of the equipment was in working order, a regular inspection schedule, retirement protocol, and qualified staff were in charge of the health and safety of the hardware, barring any further information, the recommendation I would make is that the two guides were in contact with each other, and there is to be only one person on the line at a time. Remove the potential hazard and then you don't have to worry about it at all.
