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The problem with Achilles tendon injuries is that they can occur at random. Essentially the rupture of the tendon tissue occurs under an explosive tensile load which can only result from a "perfect storm" of Vector pull and biomechanical overload.
Strengthening, stretching, conditioning and practicing box jump techniques may slightly reduce ones chances of rupturing the Achilles tendon but there really is no way to absolutely prevent it. It's a vulnerable area due to high loads and low blood supply (notwithstanding the mythology and lore of the Trojan warrior Achilles who died after being shot by an arrow in his heel cord).
As we age, the elasticity of our collagen changes and may become more susceptible. Tendon ruptures are fairly uncommon in children and are much more common in adults in their 40s. Achilles tendon ruptures are by far and away much more common in men than women. In fact, the overwhelming majority of the ones I've seen in women have been in CrossFit competitors only.
It hasn't been proven definitively, but it would appear that it's not the jumping per se, but the descending, landing, and rapid re- jumping (like a spring) that puts the tendon at greatest risk.
Julie Foucher, who I assume is in her twenties, tore her right Achilles' tendon doing box jumps in the 2015 CrossFit regional games. It could not have happened to a more highly conditioned athlete. There's been some discussion by about her training and that it included a relative scarcity of box jumps compared to other movements. In my opinion this is not necessarily relevant as this injury can occur in any setting. She tore her Achilles in the first set of box jumps. 
CrossFit Games standout Julie Foucher with a right Achilles tear.
Our thanks to FlexBuilding: A New Way to Move. A New Way to Live. for providing the anatomical images for this website.