That seems like quite a leap given how often complex computer systems fail without any malicious act.

Indeed. Never attribute to malice that which is adequately explained by stupidity. https://en.wikipedia.org/wiki/Hanlon's_razor

I don't see any convincing rebuttal. Why are airline systems not a juicy target? Why would a successful attack on the system not cause gigantic disruption? Why have these long-running, stable systems only recently begun failing so severely?

Yes, one is wise to attribute to incompetence over malice, but these systems are demonstrably not run by incompetents: they've been in operation for decades.

Or, you know, the load is increasing over time, and the system is failing to scale to it, and so will fail more and more frequently as the load continues to increase?

The amount of daily UAL passenger seats doesn't change or grow radically over weeks or months. What is the scale-up here? Everyone using their phones to check rez/status/boarding passes?

Some quick googling suggests they take delivery of several-many dozen planes a year (based on various order/delivery/etc. coverage), which would absolutely cause a step function in the number of passenger seats.

I expect there's growth on all of these:

- Total planes

- Total flights

- Number of routes

- Number of passengers

- Passenger utilization of electronic boarding

- Passenger utilization of reservation modification (seat changes, upgrades, etc.)

- Passenger utilization of in-flight electronic amenities (wifi, in flight entertainment, etc.) that are billed through the system (United has a proprietary WiFi that's tied to your frequent flier account)

- Online booking

- Travel agent/reseller pricing queries

- Travel agent/reseller booking

I'm not saying any of these are dramatically increasing, but I'd bet they're all going up slowly, which will add more and more load to the system.

Capacity only increased 0.01% over the first quarter of 2015. Passenger loads have remained flat at 81.1%. I would suggest that compared to last year, there hasn't been much change in scale. Also, all of the above systems are not linked together so I'm not sure an increase in one of those factors would be enough to take down and entire system. I could be wrong, I am definitely not an aviation IT expert. (Apparently Untied could use some though, so perhaps I might need to think about adding that to my skills!)

source: http://www.businesstravelnews.com/Travel-Management/Drop-In-...