One the characters come from a species that has pon farr and they've just entered their cycle.

The Wikipedia definition for those who aren't into Star Trek is:
"Every seven years, Vulcan males and females become aroused. They undergo a blood fever, become violent, and finally die unless they mate with someone with whom they are emphatically bonded or engage in the ritual battle known as kal-if-fee."

https://en.wikipedia.org/wiki/Pon_farr