Objective. To investigate a range of transient risk factors for an episode of sudden-onset, acute low back pain (LBP). Methods. This case-crossover study recruited 999 subjects with a new episode of acute LBP between October 2011 and November 2012 from 300 primary care clinics in Sydney, Australia. Each participant was asked to report exposure to 12 putative triggers over the 96 hours preceding the onset of back pain. Conditional logistic regression was used to estimate odds ratios (ORs) expressing the magnitude of increased risk with exposure to each trigger. Results. Exposure to a range of physical and psychosocial triggers significantly increased the risk of a new onset of LBP; ORs ranged from 2.7 (moderate or vigorous physical activity) to 25.0 (distracted during an activity or task). Age moderated the effect of exposure to heavy loads and sexual activity. The ORs for heavy loads for people ages 20, 40, or 60 years were 13.6, 6.0, and 2.7, respectively. The risk of developing back pain was greatest between 7:00 AM and noon. Conclusion. Transient exposure to a number of modifiable physical and psychosocial triggers substantially increases risk for a new episode of LBP. Triggers previously evaluated in occupational injury studies, but never in LBP, have been shown to significantly increase risk. These results aid our understanding of the causes of LBP and can inform the development of new prevention approaches.