This really intrigues me so I did a bit of googling. Here's a link to a study that was done. The conclusion from the authors:
CONCLUSIONS Atmospheric pressure reduction causes predictable, unintended insulin delivery in pumps by bubble formation and expansion of existing bubbles.
A 10-year-old girl with well-controlled type 1 diabetes managed with insulin pump therapy developed recurrent hypoglycemia 60 to 90 min after commencing commercial air travel. Upon direct inquiry, we are now aware of 50 children and adults using insulin pumps who experience this phenomenon.
In 1994, Aanderud et al. (1) demonstrated that insulin pumps delivered “more insulin than the set rate during decompression.” At sea level, the ambient pressure is 760 mmHg (1 atmosphere). When commercial planes ascend to 40,000 feet, the cabin pressure decreases by 200 mmHg to 560 mmHg (cabin pressure equivalent of 8,000 feet) (2). The objective of this study was to investigate the effects of pressure changes during airplane flight on insulin pump delivery.