According to a 1992 study, bad dreams are more likely to occur following a stressful event.

This studied examined the percentage of subjects who experienced a bad dream after the 1989 Bay Area earthquake. Those who lived closest to the epicenter and experienced the most stress had the most nightmares. Those who lived furthest from the epicenter and did not feel as stressed experienced the fewest nightmares.

Thus, the intensity of stress is positively associated with the number of nightmares.

Wood, J. M., Bootzin, R. R., Rosenhan, D., Nolen-Hoeksema, S., & Jourden, F. (1992). Effects of the 1989 san francisco earthquake on frequency and content of nightmares. Journal of Abnormal Psychology, 101(2), 219-224. doi:http://dx.doi.org/10.1037/0021-843X.101.2.219