Without checking the article, I'd guess that it's because men are more likely to be too stubborn to seek shelter when lightning is pending.

(checks article)

Yup, nailed it.

See also "Paleontologists Are Trying to Understand Why the Fossil Record Is Mostly Males" from a few days ago.

https://news.ycombinator.com/item?id=31036817

The lightning strike gender-bias is an example of the same phenomenon from a different species of placental mammals.