As Reuters notes, a similar outage occurred at the same time last year, so it might be that the customs systems were slammed with Christmas travelers and couldn't handle the excess traffic. Agents were still able to process passengers using an alternative system, albeit at a much slower rate.

Passengers depend heavily on some pretty wonky airport tech. Many airlines use ancient reservation systems, and even new ones like the Amadeus Altea system can go down, causing airport chaos. Despite those inconveniences, let's not forget that 2017 was the safest year for flying ever, with zero fatalities on scheduled passenger jets.